For journalled data, folio migration currently works by writing the folio
back, freeing the folio and faulting the new folio back in.  We can
bypass that by telling the migration code to migrate the buffer_heads
attached to our folios.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>