afs: Fix the dynamic root's d_delete to always delete unused dentries

Fix the afs dynamic root's d_delete function to always delete unused
dentries rather than only deleting them if they're positive.  With things
as they stand upstream, negative dentries stemming from failed DNS lookups
stick around preventing retries.

Fixes: 66c7e1d3 ("afs: Split the dynroot stuff out and give it its own ops tables")
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Markus Suvanto <markus.suvanto@gmail.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org