fs: fix a posible leak of allocated superblock

We probably need to fix superblock leak in patch (v4 "fs: Add user
namesapace member to struct super_block"):

Imagine posible code path in sget_userns: we iterate through
type->fs_supers and do not find suitable sb, we drop sb_lock to
allocate s and go to retry. After we dropped sb_lock some other
task from different userns takes sb_lock, it is already in retry
stage and has s allocated, so it puts its s in type->fs_supers
list. So in retry we will find these sb in list and check it has
a different userns, and finally we will return without freeing s.
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>

fs/super.c

@@ -474,6 +474,10 @@ struct super_block *sget_userns(struct file_system_type *type,
 				continue;
 			if (user_ns != old->s_user_ns) {
 				spin_unlock(&sb_lock);
+				if (s) {
+					up_write(&s->s_umount);
+					destroy_super(s);
+				}
 				return ERR_PTR(-EBUSY);
 			}
 			if (!grab_super(old))