gfs2: Fix "Introduce flag for glock holder auto-demotion"

Function demote_incompat_holders iterates over the list of glock holders
with list_for_each_entry, and it then sometimes removes the current
holder from the list.  This will get the loop stuck; we must use
list_for_each_entry_safe instead.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/glock.c

@@ -411,14 +411,14 @@ static void do_error(struct gfs2_glock *gl, const int ret)
 static void demote_incompat_holders(struct gfs2_glock *gl,
 				    struct gfs2_holder *new_gh)
 {
-	struct gfs2_holder *gh;
+	struct gfs2_holder *gh, *tmp;
 
 	/*
 	 * Demote incompatible holders before we make ourselves eligible.
 	 * (This holder may or may not allow auto-demoting, but we don't want
 	 * to demote the new holder before it's even granted.)
 	 */
-	list_for_each_entry(gh, &gl->gl_holders, gh_list) {
+	list_for_each_entry_safe(gh, tmp, &gl->gl_holders, gh_list) {
 		/*
 		 * Since holders are at the front of the list, we stop when we
 		 * find the first non-holder.