fs: dlm: move kref_put assert for lkb structs

The unhold_lkb() function decrements the lock's kref, and
asserts that the ref count was not the final one.  Use the
kref_put release function (which should not be called) to
call the assert, rather than doing the assert based on the
kref_put return value.  Using kill_lkb() as the release
function doesn't make sense if we only want to assert.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>