check_for_nsfs_mounts(): no need to take locks

check_for_nsfs_mounts(): no need to take locks

Currently we are taking mount_writer; what that function needs is
either mount_locked_reader (we are not changing anything, we just
want to iterate through the subtree) or namespace_shared and
a reference held by caller on the root of subtree - that's also
enough to stabilize the topology.

The thing is, all callers are already holding at least namespace_shared
as well as a reference to the root of subtree.

Let's make the callers provide locking warranties - don't mess with
mount_lock in check_for_nsfs_mounts() itself and document the locking
requirements.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>