linux: add rtnetlink mcast group definitions

These come in two flavors:

- RTNLGRP_* are bit indexes (arguments to setsockopt(2)) as used
  in the kernel with test_bit() but also userspace (see libnl
  examples).

- RTMGRP_* are bitmasks not used in the kernel; their use
  seems to be deprecated, at least according to comments in
  libnl, but documentation still references them.

The rationale for adding these definitions is that they're needed
to subscribe to kernel events via Netlink multicast groups.

libc-test/build.rs

@@ -3208,6 +3208,13 @@ fn test_linux(target: &str) {
             | "NFULA_VLAN_UNSPEC" // v5.4+
             | "NFULA_VLAN_PROTO" // v5.4+
             | "NFULA_VLAN_TCI" => true, // v5.4+
+            | "RTNLGRP_BRVLAN" // linux v5.6+
+            | "RTNLGRP_MCTP_IFADDR" // linux v5.17+
+            | "RTNLGRP_TUNNEL" // linux v5.18+
+            | "RTNLGRP_STATS" // linux v5.18+
+            | "RTNLGRP_MAX" // bumped with every addition
+            | "__RTNLGRP_MAX" // bumped with every addition
+                => true,
 
             _ => false,
         }

src/unix/linux_like/linux/mod.rs

@@ -2669,6 +2669,64 @@ pub const ARPD_LOOKUP: ::c_ushort = 0x02;
 pub const ARPD_FLUSH: ::c_ushort = 0x03;
 pub const ATF_MAGIC: ::c_int = 0x80;
 
+// userspace compat definitions for RTNLGRP_*
+pub const RTMGRP_LINK: ::c_int = 0x00001;
+pub const RTMGRP_NOTIFY: ::c_int = 0x00002;
+pub const RTMGRP_NEIGH: ::c_int = 0x00004;
+pub const RTMGRP_TC: ::c_int = 0x00008;
+pub const RTMGRP_IPV4_IFADDR: ::c_int = 0x00010;
+pub const RTMGRP_IPV4_MROUTE: ::c_int = 0x00020;
+pub const RTMGRP_IPV4_ROUTE: ::c_int = 0x00040;
+pub const RTMGRP_IPV4_RULE: ::c_int = 0x00080;
+pub const RTMGRP_IPV6_IFADDR: ::c_int = 0x00100;
+pub const RTMGRP_IPV6_MROUTE: ::c_int = 0x00200;
+pub const RTMGRP_IPV6_ROUTE: ::c_int = 0x00400;
+pub const RTMGRP_IPV6_IFINFO: ::c_int = 0x00800;
+pub const RTMGRP_DECnet_IFADDR: ::c_int = 0x01000;
+pub const RTMGRP_DECnet_ROUTE: ::c_int = 0x04000;
+pub const RTMGRP_IPV6_PREFIX: ::c_int = 0x20000;
+
+// enum rtnetlink_groups
+pub const RTNLGRP_NONE: ::c_uint = 0x00;
+pub const RTNLGRP_LINK: ::c_uint = 0x01;
+pub const RTNLGRP_NOTIFY: ::c_uint = 0x02;
+pub const RTNLGRP_NEIGH: ::c_uint = 0x03;
+pub const RTNLGRP_TC: ::c_uint = 0x04;
+pub const RTNLGRP_IPV4_IFADDR: ::c_uint = 0x05;
+pub const RTNLGRP_IPV4_MROUTE: ::c_uint = 0x06;
+pub const RTNLGRP_IPV4_ROUTE: ::c_uint = 0x07;
+pub const RTNLGRP_IPV4_RULE: ::c_uint = 0x08;
+pub const RTNLGRP_IPV6_IFADDR: ::c_uint = 0x09;
+pub const RTNLGRP_IPV6_MROUTE: ::c_uint = 0x0a;
+pub const RTNLGRP_IPV6_ROUTE: ::c_uint = 0x0b;
+pub const RTNLGRP_IPV6_IFINFO: ::c_uint = 0x0c;
+pub const RTNLGRP_DECnet_IFADDR: ::c_uint = 0x0d;
+pub const RTNLGRP_NOP2: ::c_uint = 0x0e;
+pub const RTNLGRP_DECnet_ROUTE: ::c_uint = 0x0f;
+pub const RTNLGRP_DECnet_RULE: ::c_uint = 0x10;
+pub const RTNLGRP_NOP4: ::c_uint = 0x11;
+pub const RTNLGRP_IPV6_PREFIX: ::c_uint = 0x12;
+pub const RTNLGRP_IPV6_RULE: ::c_uint = 0x13;
+pub const RTNLGRP_ND_USEROPT: ::c_uint = 0x14;
+pub const RTNLGRP_PHONET_IFADDR: ::c_uint = 0x15;
+pub const RTNLGRP_PHONET_ROUTE: ::c_uint = 0x16;
+pub const RTNLGRP_DCB: ::c_uint = 0x17;
+pub const RTNLGRP_IPV4_NETCONF: ::c_uint = 0x18;
+pub const RTNLGRP_IPV6_NETCONF: ::c_uint = 0x19;
+pub const RTNLGRP_MDB: ::c_uint = 0x1a;
+pub const RTNLGRP_MPLS_ROUTE: ::c_uint = 0x1b;
+pub const RTNLGRP_NSID: ::c_uint = 0x1c;
+pub const RTNLGRP_MPLS_NETCONF: ::c_uint = 0x1d;
+pub const RTNLGRP_IPV4_MROUTE_R: ::c_uint = 0x1e;
+pub const RTNLGRP_IPV6_MROUTE_R: ::c_uint = 0x1f;
+pub const RTNLGRP_NEXTHOP: ::c_uint = 0x20;
+pub const RTNLGRP_BRVLAN: ::c_uint = 0x21;
+pub const RTNLGRP_MCTP_IFADDR: ::c_uint = 0x22;
+pub const RTNLGRP_TUNNEL: ::c_uint = 0x23;
+pub const RTNLGRP_STATS: ::c_uint = 0x24;
+pub const __RTNLGRP_MAX: ::c_uint = 0x25;
+pub const RTNLGRP_MAX: ::c_uint = 0x24;
+
 // linux/module.h
 pub const MODULE_INIT_IGNORE_MODVERSIONS: ::c_uint = 0x0001;
 pub const MODULE_INIT_IGNORE_VERMAGIC: ::c_uint = 0x0002;