fs: dlm: add missing spin_unlock

This patch fixes commit dc52cd2e ("fs: dlm: fix F_CANCELLK to cancel
pending request") that we don't unlock the ops_lock in a rate case when
a waiter cannot be found. This case can only happen when cancellation of
plock operation was successful but no kernel waiter was being found.

Fixes: dc52cd2e ("fs: dlm: fix F_CANCELLK to cancel pending request")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>