afs: Fix unlink

Repeating creation and deletion of a file on an afs mount will run the box
out of memory, e.g.:

	dd if=/dev/zero of=/afs/scratch/m0 bs=$((1024*1024)) count=512
	rm /afs/scratch/m0

The problem seems to be that it's not properly decrementing the nlink count
so that the inode can be scrapped.

Note that this doesn't fix local creation followed by remote deletion.
That's harder to handle and will require a separate patch as we're not told
that the file has been deleted - only that the directory has changed.

Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>