ttm: Call shmem_writeout() from ttm_backup_backup_page()

The ->writepage operation is being removed.  Since this function
exclusively deals with shmem folios, we can call shmem_writeout()
to write it.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250402150005.2309458-8-willy@infradead.org


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