hfs: don't allow mounting over .../rsrc

That's one case when unlink() destroys a subtree, thanks to "resource
fork" idiocy.  We might forcibly evict that shit on unlink(2), but
for now let's just disallow overmounting; as it is, anything that
plays games with those would leak mounts.

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