dm integrity: use kvfree for kvmalloc'd memory

Use kvfree instead of kfree because the array is allocated with kvmalloc.

Fixes: 7eada909 ("dm: add integrity target")
Cc: stable@vger.kernel.org # v4.12+
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>