btrfs: refactor end_bio_extent_readpage code flow

Untangle the goto and move the code it jumps to so it goes in the order
of the most likely states first.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>