dm integrity: allow unaligned bv_offset

dm integrity: allow unaligned bv_offset

When slub_debug is enabled kmalloc returns unaligned memory. XFS uses
this unaligned memory for its buffers (if an unaligned buffer crosses a
page, XFS frees it and allocates a full page instead - see the function
xfs_buf_allocate_memory).

dm-integrity checks if bv_offset is aligned on page size and this check
fail with slub_debug and XFS.

Fix this bug by removing the bv_offset check, leaving only the check for
bv_len.

Fixes: 7eada909bfd7 ("dm: add integrity target")
Cc: [email protected] # v4.12+
Reported-by: Bruno Prémont <[email protected]>
Reviewed-by: Milan Broz <[email protected]>
Signed-off-by: Mikulas Patocka <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>