btrfs: simplify compressed/inline check in __extent_writepage_io()

Commit 7087a9d8 ("btrfs: Remove
extent_io_ops::writepage_end_io_hook") left this logic in a confusing
state. Simplify it.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>