shmem: Add shmem_writeout()

This will be the replacement for shmem_writepage().

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250402150005.2309458-6-willy@infradead.org


Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>