btrfs: introduce helper to handle page status update in end_bio_extent_readpage()

Introduce a new helper to handle update page status in
end_bio_extent_readpage(). This will be later used for subpage support
where the page status update can be more complex than now.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>