gfs2: Add a migrate_folio operation for journalled files

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) <[email protected]>
Signed-off-by: Andreas Gruenbacher <[email protected]>

Author: Matthew Wilcox (Oracle)

fs/gfs2/aops.c

@@ -116,8 +116,7 @@ static int gfs2_write_jdata_folio(struct folio *folio,
  * @folio: The folio to write
  * @wbc: The writeback control
  *
- * This is shared between writepage and writepages and implements the
- * core of the writepage operation. If a transaction is required then
+ * Implements the core of write back. If a transaction is required then
  * the checked flag will have been set and the transaction will have
  * already been started before this is called.
  */
@@ -755,6 +754,7 @@ static const struct address_space_operations gfs2_jdata_aops = {
 	.readahead = gfs2_readahead,
 	.dirty_folio = jdata_dirty_folio,
 	.bmap = gfs2_bmap,
+	.migrate_folio = buffer_migrate_folio,
 	.invalidate_folio = gfs2_invalidate_folio,
 	.release_folio = gfs2_release_folio,
 	.is_partially_uptodate = block_is_partially_uptodate,