ceph: do not skip the first folio of the next object in writeback

When `ceph_process_folio_batch` encounters a folio past the end of the
current object, it should leave it in the batch so that it is picked up
in the next iteration.

Removing the folio from the batch means that it does not get written
back and remains dirty instead. This makes `fsync()` silently skip some
of the data, delays capability release, and breaks coherence with
`O_DIRECT`.

The link below contains instructions for reproducing the bug.

Cc: [email protected]
Fixes: ce80b76 ("ceph: introduce ceph_process_folio_batch() method")
Link: https://tracker.ceph.com/issues/75156
Signed-off-by: Hristo Venev <[email protected]>
Reviewed-by: Viacheslav Dubeyko <[email protected]>
Signed-off-by: Ilya Dryomov <[email protected]>

Author: hvenev

fs/ceph/addr.c

@@ -1326,7 +1326,6 @@ void ceph_process_folio_batch(struct address_space *mapping,
 			continue;
 		} else if (rc == -E2BIG) {
 			folio_unlock(folio);
-			ceph_wbc->fbatch.folios[i] = NULL;
 			break;
 		}