[GFS2] panic after can't parse mount arguments

When you try to mount gfs2 with -o garbage, the mount fails and the gfs2
superblock is deallocated and becomes NULL. The vfs comes around later
on and calls gfs2_kill_sb. At this point the hidden gfs2 superblock
pointer (sb->s_fs_info) is NULL and dereferencing it through
gfs2_meta_syncfs causes the panic. (the other function call to
gfs2_delete_debugfs_file() succeeds because this function already checks
for a NULL pointer)
Signed-off-by: NAbhijith Das <adas@redhat.com>
Signed-off-by: NSteven Whitehouse <swhiteho@redhat.com>

fs/gfs2/ops_fstype.c

@@ -887,8 +887,10 @@ static int gfs2_get_sb_meta(struct file_system_type *fs_type, int flags,
 
 static void gfs2_kill_sb(struct super_block *sb)
 {
-	gfs2_delete_debugfs_file(sb->s_fs_info);
-	gfs2_meta_syncfs(sb->s_fs_info);
+	if (sb->s_fs_info) {
+		gfs2_delete_debugfs_file(sb->s_fs_info);
+		gfs2_meta_syncfs(sb->s_fs_info);
+	}
 	kill_block_super(sb);
 }