fs: fixes from testing

fs: fixes from testing

1. Don't allow rmdir .
2. Don't allow rmdir of a directory with a file in it (we stopped only subdirs)
3. Don't consider unreferenced inodes when checking if a remount is valid
4. Fix the case of

	process A
		cd /tmp/foo
	process B
		rmdir /tmp/foo
	process A
		ls .   (stale entry)