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)

fs/afs/write.c

@@ -784,7 +784,7 @@ int afs_writepages(struct address_space *mapping,
 	} else if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX) {
 		ret = afs_writepages_region(mapping, wbc, 0, LLONG_MAX, &next);
 		if (wbc->nr_to_write > 0 && ret == 0)
-			mapping->writeback_index = next;
+			mapping->writeback_index = next / PAGE_SIZE;
 	} else {
 		ret = afs_writepages_region(mapping, wbc,
 					    wbc->range_start, wbc->range_end, &next);