fs: dlm: check for pending users filling buffers

Currently we don't care if the DLM application stack is filling buffers
(not committed yet) while we transmit some already committed buffers.
By checking on active writequeue users before dequeue a writequeue entry
we know there is coming more data and do nothing. We wait until the send
worker will be triggered again if the writequeue entry users hit zero.

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