Minor diff reduction with ZoF in include/sys

- move linux/ includes to platform headers
- add void * io_bio to zio for tracking the underlying bio
- add freebsd specific fields to abd_scatter

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9615

include/sys/zio.h

@@ -500,6 +500,7 @@ struct zio {
 	zio_gang_node_t	*io_gang_tree;
 	void		*io_executor;
 	void		*io_waiter;
+	void		*io_bio;
 	kmutex_t	io_lock;
 	kcondvar_t	io_cv;
 	int		io_allocator;