afs: Fix setting of writeback_index

Fix afs_writepages() to always set mapping->writeback_index to a page index
and not a byte position[1].

Fixes: 31143d5d ("AFS: implement basic file write support")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/CAB9dFdvHsLsw7CMnB+4cgciWDSqVjuij4mH3TaXnHQB8sz5rHw@mail.gmail.com/ [1]
Link: https://lore.kernel.org/r/162610728339.3408253.4604750166391496546.stgit@warthog.procyon.org.uk/ # v2 (no v1)