Fix userland build by making io_task available only for the kernel and by

providing taskq_dispatch_safe() macro.

MFC after:	1 week

cddl/contrib/opensolaris/lib/libzpool/common/sys/zfs_context.h

@@ -343,6 +343,9 @@ extern void	taskq_wait(taskq_t *);
 extern int	taskq_member(taskq_t *, void *);
 extern void	system_taskq_init(void);
 
+#define	taskq_dispatch_safe(tq, func, arg, task)			\
+	taskq_dispatch((tq), (func), (arg), TQ_SLEEP)
+
 #define	XVA_MAPSIZE	3
 #define	XVA_MAGIC	0x78766174
 

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio.h

@@ -317,8 +317,10 @@ struct zio {
 	/* FMA state */
 	uint64_t	io_ena;
 
+#ifdef _KERNEL
 	/* FreeBSD only. */
 	struct ostask	io_task;
+#endif
 };
 
 extern zio_t *zio_null(zio_t *pio, spa_t *spa,