path_is_under(): use guards

path_is_under(): use guards

... and document that locking requirements for is_path_reachable().
There is one questionable caller in do_listmount() where we are not
holding mount_lock *and* might not have the first argument mounted.
However, in that case it will immediately return true without having
to look at the ancestors.  Might be cleaner to move the check into
non-LSTM_ROOT case which it really belongs in - there the check is
not always true and is_mounted() is guaranteed.

Document the locking environments for is_path_reachable() callers:
	get_peer_under_root()
	get_dominating_id()
	do_statmount()
	do_listmount()

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