dm integrity: fix error code in dm_integrity_ctr()

The "r" variable shadows an earlier "r" that has function scope.  It
means that we accidentally return success instead of an error code.
Smatch has a warning for this:

	drivers/md/dm-integrity.c:4503 dm_integrity_ctr()
	warn: missing error code 'r'

Fixes: 7eada909 ("dm: add integrity target")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>