The core is to convert direct page usage into sector_ptr usage, and
use memcpy() to replace copy_page().

For pointers usage, we need to convert it to kmap_local_page() +
sector->pgoff.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>