btrfs: simplify main loop in submit_extent_page

bio_add_page always adds either the entire range passed to it or nothing.
Based on that btrfs_bio_add_page can only return a length smaller than
the passed in one when hitting the ordered extent limit, which can only
happen for writes.  Given that compressed writes never even use this code
path, this means that all the special cases for compressed extent offset
handling are dead code.

Reflow submit_extent_page to take advantage of this by inlining
btrfs_bio_add_page and handling the ordered extent limit by decrementing
it for each added range and thus significantly simplifying the loop.

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>