follow_dotdot{,_rcu}(): massage loops

The logics in both of them is the same:
	while true
		if in process' root	// uncommon
			break
		if *not* in mount root	// normal case
			find the parent
			return
		if at absolute root	// very uncommon
			break
		move to underlying mountpoint
	report that we are in root

Pull the common path out of the loop:
	if in process' root		// uncommon
		goto in_root
	if unlikely(in mount root)
		while true
			if at absolute root
				goto in_root
			move to underlying mountpoint
			if in process' root
				goto in_root
			if in mount root
				break;
	find the parent	// we are not in mount root
	return
in_root:
	report that we are in root

The reason for that transformation is that we get to keep the
common path straight *and* get a separate block for "move
through underlying mountpoints", which will allow to sanitize
NO_XDEV handling there.  What's more, the pared-down loops
will be easier to deal with - in particular, non-RCU case
has no need to grab mount_lock and rewriting it to the
form that wouldn't do that is a non-trivial change.  Better
do that with less stuff getting in the way...

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