9p: Add a migrate_folio method

commit 03ddd77 upstream.

The migration code used to be able to migrate dirty 9p folios by writing
them back using writepage.  When the writepage method was removed,
we neglected to add a migrate_folio method, which means that dirty 9p
folios have been unmovable ever since.  This reduced our success at
defragmenting memory on machines which use 9p heavily.

Fixes: 80105ed (9p: Use netfslib read/write_iter)
Cc: [email protected]
Cc: David Howells <[email protected]>
Cc: [email protected]
Signed-off-by: "Matthew Wilcox (Oracle)" <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Acked-by: Dominique Martinet <[email protected]>
Reviewed-by: David Howells <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Matthew Wilcox (Oracle)

fs/9p/vfs_addr.c

@@ -160,4 +160,5 @@ const struct address_space_operations v9fs_addr_operations = {
 	.invalidate_folio	= netfs_invalidate_folio,
 	.direct_IO		= noop_direct_IO,
 	.writepages		= netfs_writepages,
+	.migrate_folio		= filemap_migrate_folio,
 };