[DLM] Fix spin lock already unlocked bug

I just noticed this message when testing some other changes I'd made to
lowcomms (to use workqueues) but the problem seems to be in the current
git trees too. I'm amazed no-one has seen it.

    BUG: spinlock already unlocked on CPU#1, dlm_recoverd/16868
Signed-Off-By: NPatrick Caulfield <pcaulfie@redhat.com>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>

fs/dlm/lowcomms-tcp.c

@@ -709,6 +709,7 @@ void *dlm_lowcomms_get_buffer(int nodeid, int len,
 	if (!con)
 		return NULL;
 
+	spin_lock(&con->writequeue_lock);
 	e = list_entry(con->writequeue.prev, struct writequeue_entry, list);
 	if ((&e->list == &con->writequeue) ||
 	    (PAGE_CACHE_SIZE - e->end < len)) {
@@ -747,6 +748,7 @@ void dlm_lowcomms_commit_buffer(void *mh)
 	struct connection *con = e->con;
 	int users;
 
+	spin_lock(&con->writequeue_lock);
 	users = --e->users;
 	if (users)
 		goto out;