propagate_one(): mnt_set_mountpoint() needs mount_lock

... to protect the modification of mp->m_count done by it.  Most of
the places that modify that thing also have namespace_lock held,
but not all of them can do so, so we really need mount_lock here.
Kudos to Piotr Krysiuk <piotras@gmail.com>, who'd spotted a related
bug in pivot_root(2) (fixed unnoticed in 5.3); search for other
similar turds has caught out this one.

Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/pnode.c

@@ -261,14 +261,13 @@ static int propagate_one(struct mount *m)
 	child = copy_tree(last_source, last_source->mnt.mnt_root, type);
 	if (IS_ERR(child))
 		return PTR_ERR(child);
+	read_seqlock_excl(&mount_lock);
 	mnt_set_mountpoint(m, mp, child);
+	if (m->mnt_master != dest_master)
+		SET_MNT_MARK(m->mnt_master);
+	read_sequnlock_excl(&mount_lock);
 	last_dest = m;
 	last_source = child;
-	if (m->mnt_master != dest_master) {
-		read_seqlock_excl(&mount_lock);
-		SET_MNT_MARK(m->mnt_master);
-		read_sequnlock_excl(&mount_lock);
-	}
 	hlist_add_head(&child->mnt_hash, list);
 	return count_mounts(m->mnt_ns, child);
 }