erofs: clean up preload_compressed_pages()

Rename preload_compressed_pages() as z_erofs_bind_cache()
since we're trying to prepare for adapting folios.

Also, add a comment for the gfp setting. No logic changes.

Link: https://lore.kernel.org/r/20220301194951.106227-2-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>