9p: Add a migrate_folio method

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]>

Author: Matthew Wilcox (Oracle)

fs/9p/vfs_addr.c

@@ -164,4 +164,5 @@ 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,
 };