afs: Fix unlink to handle YFS.RemoveFile2 better

Make use of the status update for the target file that the YFS.RemoveFile2
RPC op returns to correctly update the vnode as to whether the file was
actually deleted or just had nlink reduced.

Fixes: 30062bd1

 ("afs: Implement YFS support in the fs client")
Signed-off-by: David Howells <dhowells@redhat.com>