block: Disable -Walign-mismatch for blk-mq.c

LLVM 13 adds a new warning, -Walign-mismatch, which has an instance in
blk_mq_complete_send_ipi():

block/blk-mq.c:630:39: warning: passing 8-byte aligned argument to
32-byte aligned parameter 2 of 'smp_call_function_single_async' may
result in an unaligned pointer access [-Walign-mismatch]
                smp_call_function_single_async(cpu, &rq->csd);
                                                    ^
1 warning generated.

This is expected after commit 4ccafe0 ("block: unalign
call_single_data in struct request"), which purposefully unaligned the
structure to save space. Given that there is no real alignment
requirement and there have been no reports of issues since that change,
it should be safe to disable the warning for this one translation unit.

Link: ClangBuiltLinux/linux#1328
Link: https://lore.kernel.org/r/20210310182307.zzcbi5w5jrmveld4@archlinux-ax161/
Link: https://lore.kernel.org/r/20210330230249.709221-1-jiancai@google.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210408194458.501617-1-nathan@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

block/Makefile

@@ -10,6 +10,7 @@ obj-$(CONFIG_BLOCK) := bio.o elevator.o blk-core.o blk-sysfs.o \
 			blk-mq-sysfs.o blk-mq-cpumap.o blk-mq-sched.o ioctl.o \
 			genhd.o ioprio.o badblocks.o partitions/ blk-rq-qos.o
 
+CFLAGS_blk-mq.o := $(call cc-disable-warning, align-mismatch)
 obj-$(CONFIG_BOUNCE)		+= bounce.o
 obj-$(CONFIG_BLK_SCSI_REQUEST)	+= scsi_ioctl.o
 obj-$(CONFIG_BLK_DEV_BSG)	+= bsg.o