migrate: Remove call to ->writepage

The writepage callback is going away; filesystems must implement
migrate_folio or else dirty folios will not be migratable.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250402150005.2309458-4-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>