hfsplus: fix rename over directories

When renaming over a directory we need to use hfsplus_rmdir instead of
hfsplus_unlink to evict the victim.  This makes sure we properly error out
on non-empty directory as required by Posix (BZ #16571), and it also makes
sure we do the right thing in case i_nlink will every be set correctly for
directories on hfsplus.

Reported-by: Vlado Plaga <rechner@vlado-do.de>
Signed-off-by: Christoph Hellwig <hch@tuxera.com>