HFS+: fix unlink of links

upstream commit: 76b0c26a

Some time ago while attempting to handle invalid link counts, I botched 
the unlink of links itself, so this patch fixes this now correctly, so 
that only the link count of nodes that don't point to links is ignored.
Thanks to Vlado Plaga <rechner@vlado-do.de> to notify me of this 
problem.

Signed-off-by: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>