[GFS2] check kthread_should_stop when waiting

Use wait_event_interruptible() in the lock_dlm thread instead
of an open coded equivalent, and include a kthread_should_stop()
check in the wait test so we don't miss a kthread_stop().

Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>