null_blk: unalign call_single_data

Commit 966a9671 randomly added alignment to this structure, but
it's actually detrimental to performance of null_blk. Test case:

Running on both the home and remote node shows a ~5% degradation
in performance.

While in there, move blk_status_t to the hole after the integer tag
in the nullb_cmd structure. After this patch, we shrink the size
from 192 to 152 bytes.

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