fs: dlm: fix F_CANCELLK to cancel pending request

This patch fixes the current handling of F_CANCELLK by not just doing a
unlock as we need to try to cancel a lock at first. A unlock makes sense
on a non-blocking lock request but if it's a blocking lock request we
need to cancel the request until it's not granted yet. This patch is fixing
this behaviour by first try to cancel a lock request and if it's failed
it's unlocking the lock which seems to be granted.

Note: currently the nfs locking handling was disabled by commit
40595cdc

 ("nfs: block notification on fs with its own ->lock").
However DLM was never being updated regarding to this change. Future
patches will try to fix lockd lock requests for DLM. This patch is
currently assuming the upstream DLM lockd handling is correct.

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