dm integrity: fix data corruption due to improper use of bvec_kmap_local

Commit 25058d1c ("dm integrity: use bvec_kmap_local in
__journal_read_write") didn't account for __journal_read_write() later
adding the biovec's bv_offset. As such using bvec_kmap_local() caused
the start of the biovec to be skipped.

Trivial test that illustrates data corruption:

  # integritysetup format /dev/pmem0
  # integritysetup open /dev/pmem0 integrityroot
  # mkfs.xfs /dev/mapper/integrityroot
  ...
  bad magic number
  bad magic number
  Metadata corruption detected at xfs_sb block 0x0/0x1000
  libxfs_writebufr: write verifer failed on xfs_sb bno 0x0/0x1000
  releasing dirty buffer (bulk) to free list!

Fix this by using kmap_local_page() instead of bvec_kmap_local() in
__journal_read_write().

Fixes: 25058d1c ("dm integrity: use bvec_kmap_local in __journal_read_write")
Reported-by: Tony Asleson <tasleson@redhat.com>
Reviewed-by: Heinz Mauelshagen <heinzm@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>