configs: Enable MPTCP support

The official description says:

  Multipath TCP (MPTCP) connections send and receive data over multiple
  subflows in order to utilize multiple network paths. Each subflow uses
  the TCP protocol, and TCP options carry header information for MPTCP.

In other words, we can easily use multiple Internet links at the same
time and for the same connection. For example, we can plug a 4G/5G USB
dongle in a RPi and use both the WiFi and the cellular networks to get
more speed, more redundancy, etc.

An example of use with a RPi4 and Ubuntu 22.04:

  https://www.tessares.net/mptcp-proxy-setup-at-home-easier-now/

MPTCP support is light and enabled in most Linux distributions today:
RHEL, Debian, Ubuntu, Fedora, ArchLinux, etc.

Note that to use MPTCP with IPv6 addresses, it is required to have IPV6
not built as a module, see commit 0ed37ac ("mptcp: depends on IPV6
but not as a module") for more details. When talking with netdev
maintainers before the introduction of MPTCP in the upstream kernel in
v5.6, it looks like there is not so much reasons to still have IPv6 as a
module as IPv6 is (supposed to be) "today's Internet". This module will
likely be quickly loaded on devices with network devices anyway.

If having IPV6 built-in is really an issue, MPTCP can still be enabled
but without IPv6 support which is not recommended today.

CONFIG_INET_MPTCP_DIAG is also enabled as a module, similar to
CONFIG_INET_DIAG.

Closes: #4254
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

arch/arm/configs/bcm2709_defconfig

@@ -102,7 +102,7 @@ CONFIG_INET_IPCOMP=m
 CONFIG_INET_DIAG=m
 CONFIG_TCP_CONG_ADVANCED=y
 CONFIG_TCP_CONG_BBR=m
-CONFIG_IPV6=m
+CONFIG_IPV6=y
 CONFIG_IPV6_ROUTER_PREF=y
 CONFIG_IPV6_ROUTE_INFO=y
 CONFIG_INET6_AH=m
@@ -118,6 +118,9 @@ CONFIG_IPV6_SUBTREES=y
 CONFIG_IPV6_MROUTE=y
 CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
 CONFIG_IPV6_PIMSM_V2=y
+CONFIG_MPTCP=y
+CONFIG_INET_MPTCP_DIAG=m
+CONFIG_MPTCP_IPV6=y
 CONFIG_NETFILTER=y
 CONFIG_BRIDGE_NETFILTER=m
 CONFIG_NF_CONNTRACK=m

arch/arm/configs/bcm2711_defconfig

@@ -102,7 +102,7 @@ CONFIG_INET_IPCOMP=m
 CONFIG_INET_DIAG=m
 CONFIG_TCP_CONG_ADVANCED=y
 CONFIG_TCP_CONG_BBR=m
-CONFIG_IPV6=m
+CONFIG_IPV6=y
 CONFIG_IPV6_ROUTER_PREF=y
 CONFIG_IPV6_ROUTE_INFO=y
 CONFIG_INET6_AH=m
@@ -118,6 +118,9 @@ CONFIG_IPV6_SUBTREES=y
 CONFIG_IPV6_MROUTE=y
 CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
 CONFIG_IPV6_PIMSM_V2=y
+CONFIG_MPTCP=y
+CONFIG_INET_MPTCP_DIAG=m
+CONFIG_MPTCP_IPV6=y
 CONFIG_NETWORK_PHY_TIMESTAMPING=y
 CONFIG_NETFILTER=y
 CONFIG_BRIDGE_NETFILTER=m

arch/arm/configs/bcmrpi_defconfig

@@ -97,7 +97,7 @@ CONFIG_INET_IPCOMP=m
 CONFIG_INET_DIAG=m
 CONFIG_TCP_CONG_ADVANCED=y
 CONFIG_TCP_CONG_BBR=m
-CONFIG_IPV6=m
+CONFIG_IPV6=y
 CONFIG_IPV6_ROUTER_PREF=y
 CONFIG_IPV6_ROUTE_INFO=y
 CONFIG_INET6_AH=m
@@ -113,6 +113,9 @@ CONFIG_IPV6_SUBTREES=y
 CONFIG_IPV6_MROUTE=y
 CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
 CONFIG_IPV6_PIMSM_V2=y
+CONFIG_MPTCP=y
+CONFIG_INET_MPTCP_DIAG=m
+CONFIG_MPTCP_IPV6=y
 CONFIG_NETFILTER=y
 CONFIG_BRIDGE_NETFILTER=m
 CONFIG_NF_CONNTRACK=m

arch/arm64/configs/bcm2711_defconfig

@@ -106,7 +106,7 @@ CONFIG_INET_IPCOMP=m
 CONFIG_INET_DIAG=m
 CONFIG_TCP_CONG_ADVANCED=y
 CONFIG_TCP_CONG_BBR=m
-CONFIG_IPV6=m
+CONFIG_IPV6=y
 CONFIG_IPV6_ROUTER_PREF=y
 CONFIG_IPV6_ROUTE_INFO=y
 CONFIG_INET6_AH=m
@@ -122,6 +122,9 @@ CONFIG_IPV6_SUBTREES=y
 CONFIG_IPV6_MROUTE=y
 CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
 CONFIG_IPV6_PIMSM_V2=y
+CONFIG_MPTCP=y
+CONFIG_INET_MPTCP_DIAG=m
+CONFIG_MPTCP_IPV6=y
 CONFIG_NETWORK_PHY_TIMESTAMPING=y
 CONFIG_NETFILTER=y
 CONFIG_BRIDGE_NETFILTER=m

arch/arm64/configs/bcmrpi3_defconfig

@@ -104,7 +104,7 @@ CONFIG_INET_IPCOMP=m
 CONFIG_INET_DIAG=m
 CONFIG_TCP_CONG_ADVANCED=y
 CONFIG_TCP_CONG_BBR=m
-CONFIG_IPV6=m
+CONFIG_IPV6=y
 CONFIG_IPV6_ROUTER_PREF=y
 CONFIG_INET6_AH=m
 CONFIG_INET6_ESP=m
@@ -118,6 +118,9 @@ CONFIG_IPV6_SUBTREES=y
 CONFIG_IPV6_MROUTE=y
 CONFIG_IPV6_MROUTE_MULTIPLE_TABLES=y
 CONFIG_IPV6_PIMSM_V2=y
+CONFIG_MPTCP=y
+CONFIG_INET_MPTCP_DIAG=m
+CONFIG_MPTCP_IPV6=y
 CONFIG_NETFILTER=y
 CONFIG_BRIDGE_NETFILTER=m
 CONFIG_NF_CONNTRACK=m