dm integrity: don't check null pointer before kvfree and vfree

The functions kfree, vfree and kvfree do nothing if we pass a NULL
pointer to them.  So we don't need to test the pointer for NULL.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>