[DLM] Fix dlm_lowcoms_stop hang

When you attempt to release a lockspace in DLM, it will hang trying to down a
semaphore that has already been downed.  The attached patch fixes the problem.

Signed-off-by: Josef Bacik <jwhiter@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Patrick Caulfield <pcaulfie@redhat.com>