[GFS2] Fix lock ordering bug in page fault path

Mmapped files were able to trigger a lock ordering bug. Private
maps do not need to take the glock so early on. Shared maps do
unfortunately, however we can get around that by adding a flag
into the flags for the struct gfs2_file. This only works because
we are taking an exclusive lock at this point, so we know that
nobody else can be racing with us.

Fixes Red Hat bugzilla: #201196

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

fs/gfs2/log.c

@@ -256,8 +256,8 @@ static unsigned int current_tail(struct gfs2_sbd *sdp)
 	if (list_empty(&sdp->sd_ail1_list))
 		tail = sdp->sd_log_head;
 	else {
-		ai = list_entry(sdp->sd_ail1_list.prev,
-				struct gfs2_ail, ai_list);
+		ai = list_entry(sdp->sd_ail1_list.prev, struct gfs2_ail,
+				ai_list);
 		tail = ai->ai_first;
 	}