ceph: remove error return from ceph_process_folio_batch()

Following an earlier commit, ceph_process_folio_batch() no longer
returns errors because the writeback loop cannot handle them.

Since this function already indicates failure to lock any pages by
leaving `ceph_wbc.locked_pages == 0`, and the writeback loop has no way
to handle abandonment of a locked batch, change the return type of
ceph_process_folio_batch() to `void` and remove the pathological goto in
the writeback loop. The lack of a return code emphasizes that
ceph_process_folio_batch() is designed to be abort-free: that is, once
it commits a folio for writeback, it will not later abandon it or
propagate an error for that folio. Any future changes requiring "abort"
logic should follow this invariant by cleaning up its array and
resetting ceph_wbc.locked_pages appropriately.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>