fs: dlm: clear pending bit when queue was empty

This patch clears the DLM_IFL_CB_PENDING_BIT flag which will be set when
there is callback work queued when there was no callback to dequeue. It
is a buggy case and should never happen, that's why there is a
WARN_ON(). However if the case happens we are prepared to somehow
recover from it.

Cc: stable@vger.kernel.org
Fixes: 61bed0ba ("fs: dlm: use a non-static queue for callbacks")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>