dm-integrity: fix a memory leak when rechecking the data

Memory for the "checksums" pointer will leak if the data is rechecked
after checksum failure (because the associated kfree won't happen due
to 'goto skip_io').

Fix this by freeing the checksums memory before recheck, and just use
the "checksum_onstack" memory for storing checksum during recheck.

Fixes: c88f5e55

 ("dm-integrity: recheck the integrity tag after a failure")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>