[GFS2] Use atomic_t rather than kref in glock.c

Use atomic_t as the ref count in glocks rather than a kref.
This is another step towards using RCU for the glock hash.

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

fs/gfs2/incore.h

@@ -164,7 +164,7 @@ struct gfs2_glock {
 	struct hlist_node gl_list;
 	unsigned long gl_flags;		/* GLF_... */
 	struct lm_lockname gl_name;
-	struct kref gl_ref;
+	atomic_t gl_ref;
 
 	spinlock_t gl_spin;