block: unalign call_single_data in struct request

A previous change blindly added massive alignment to the
call_single_data structure in struct request. This ballooned it in size
from 296 to 320 bytes on my setup, for no valid reason at all.

Use the unaligned struct __call_single_data variant instead.

Fixes: 966a9671 ("smp: Avoid using two cache lines for struct call_single_data")
Cc: stable@vger.kernel.org # v4.14
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -135,7 +135,7 @@ typedef __u32 __bitwise req_flags_t;
 struct request {
 	struct list_head queuelist;
 	union {
-		call_single_data_t csd;
+		struct __call_single_data csd;
 		u64 fifo_time;
 	};