afs: Use op->nr_iterations=-1 to indicate to begin fileserver iteration

Set op->nr_iterations to -1 to indicate that we need to begin fileserver
iteration rather than setting error to SHRT_MAX.  This makes it easier to
eliminate the address cursor.

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