staging: erofs: move stagingpage operations to compress.h

stagingpages are behaved as bounce pages for temporary use.
Move to compress.h since the upcoming decompressor will
allocate stagingpages as well.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>