dlm: convert connections_lock in a mutex

The semaphore connections_lock is used as a mutex.  Convert it to the mutex
API.

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Cc: Christine Caulfield <ccaulfie@redhat.com>
Cc: David Teigland <teigland@redhat.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Teigland <teigland@redhat.com>