btrfs: sink calc_bio_boundaries into its only caller

Nowadays calc_bio_boundaries() is a relatively simple function that only
guarantees the one bio equals to one ordered extent rule for uncompressed
Zone Append bios.

Sink it into it's only caller alloc_new_bio().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>