blk-mq: Tentetively disable -Walign-mismatch warning

This silences the following compilation warning, introduced via
current upstream LLVM toolchain builds:

../block/blk-mq.c:581:44: 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(ctx->cpu, &rq->csd);
                                                         ^
1 warning generated.

https://lore.kernel.org/linux-block/20210310225240.4epj2mdmz



Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>
Change-Id: Id2911fa6298d66d0f68a06f5a355b5a99ecd95e7