9p: Add a migrate_folio method

commit 03ddd772 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: 80105ed2 (9p: Use netfslib read/write_iter)
Cc: stable@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: v9fs@lists.linux.dev
Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250402150005.2309458-2-willy@infradead.org


Acked-by: Dominique Martinet <asmadeus@codewreck.org>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>