btrfs: remove non-standard extent handling in __extent_writepage_io

__extent_writepage_io is never called for compressed or inline extents,
or holes.  Remove the not quite working code for them and replace it with
asserts that these cases don't happen.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>