[GFS2] Use void * instead of typedef for locking module interface

As requested by Jan Engelhardt, this removes the typedefs in the
locking module interface and replaces them with void *. Also
since we are changing the interface, I've added a few consts
as well.

Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Cc: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/incore.h

@@ -187,7 +187,7 @@ struct gfs2_glock {
 	struct gfs2_holder *gl_req_gh;
 	gfs2_glop_bh_t gl_req_bh;
 
-	lm_lock_t *gl_lock;
+	void *gl_lock;
 	char *gl_lvb;
 	atomic_t gl_lvb_count;