fs: actually hold the namespace semaphore

Don't use a scoped guard that only protects the next statement.

Use a regular guard to make sure that the namespace semaphore is held
across the whole function.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Reported-by: Leon Romanovsky <leon@kernel.org>
Link: https://lore.kernel.org/all/20250401170715.GA112019@unreal/
Fixes: db04662 ("fs: allow detached mounts in clone_private_mount()")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/namespace.c

@@ -2478,7 +2478,8 @@ struct vfsmount *clone_private_mount(const struct path *path)
 	struct mount *old_mnt = real_mount(path->mnt);
 	struct mount *new_mnt;
 
-	scoped_guard(rwsem_read, &namespace_sem)
+	guard(rwsem_read)(&namespace_sem);
+
 	if (IS_MNT_UNBINDABLE(old_mnt))
 		return ERR_PTR(-EINVAL);