f2fs: add customized migrate_page callback

This patch improves the migration of dirty pages and allows migrating atomic
written pages that F2FS uses in Page Cache. Instead of the fallback releasing
page path, it provides better performance for memory compaction, CMA and other
users of memory page migrating. For dirty pages, there is no need to write back
first when migrating. For an atomic written page before committing, we can
migrate the page and update the related 'inmem_pages' list at the same time.

Signed-off-by: Weichao Guo <guoweichao@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix some coding style]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>