[GFS2] fix bz 231369, gfs2 will oops if you specify an invalid mount option

If you specify an invalid mount option when trying to mount a gfs2 filesystem,
gfs2 will oops.  The attached patch resolves this problem.
Signed-off-by: Josef Whiter <jwhiter@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/glock.c

@@ -2217,7 +2217,7 @@ int gfs2_create_debugfs_file(struct gfs2_sbd *sdp)
 
 void gfs2_delete_debugfs_file(struct gfs2_sbd *sdp)
 {
-	if (sdp->debugfs_dentry)
+	if (sdp && sdp->debugfs_dentry)
 		debugfs_remove(sdp->debugfs_dentry);
 }