btrfs: remove the force_bio_submit to submit_extent_page

If force_bio_submit, submit_extent_page simply calls submit_one_bio as
the first thing.  This can just be moved to the only caller that sets
force_bio_submit to true.

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