afs: Return ENOENT if no cell DNS record can be found

Make AFS return error ENOENT if no cell SRV or AFSDB DNS record (or
cellservdb config file record) can be found rather than returning
EDESTADDRREQ.

Also add cell name lookup info to the cursor dump.

Fixes: d5c32c89

 ("afs: Fix cell DNS lookup")
Reported-by: Markus Suvanto <markus.suvanto@gmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216637


Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org