[DLM] Convert a semaphore to a completion

Convert a semaphore into a completion in device.c.

Cc: David Teigland <teigland@redhat.com>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: Patrick Caulfield <pcaulfie@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>