block: use the request length for iov alignment

An iov length needs to be aligned to the logical block size, which may
be larger than the memory alignment.

Tested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Message-Id: <20220929200523.3218710-3-kbusch@meta.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>