dm bufio: fix buffer alignment

Commit 6b5e718c ("dm bufio: relax alignment constraint on slab
cache") relaxed alignment on dm-bufio cache, however it may break
dm-crypt or dm-integrity.

dm-crypt and dm-integrity require that the size of bio vector entries
(bv_len) is aligned on its sector size. bv_offset doesn't have to be
aligned, but bv_len must be. XFS sends unaligned bios, but they do not
cross page boundary, so the requirement for aligned bv_len is met.

Commit 6b5e718c made dm-bufio send unaligned bios that cross page
boundary, this could break dm-crypt and dm-integrity.

Reinstates the alignment. Note that misaligned entries only happen when
we use slab/slub debugging. Without debugging, the entries are always
aligned.

Fixes: 6b5e718c ("dm bufio: relax alignment constraint on slab cache")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>