fs: dlm: requestqueue busy wait to event based wait

This patch changes the requestqueue busy waiting algorithm to use
atomic counter values and wait_event() to wait until the requestqueue is
empty. It will slightly reduce the number of holding ls_requestqueue_mutex
mutex.

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