Disable LBA weighting on files and SSDs

The LBA weighting makes sense on rotational media where the outer tracks
have twice the bandwidth of the inner tracks. However, it is detrimental
on nonrotational media such as solid state disks, where the only effect
is to ensure that metaslabs enter the best-fit allocation behavior
sooner, which is detrimental to performance. It also makes no sense on
files where the underlying filesystem can arrange things however it
wants.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3712

include/sys/vdev_impl.h

@@ -151,6 +151,7 @@ struct vdev {
 	vdev_stat_t	vdev_stat;	/* virtual device statistics	*/
 	boolean_t	vdev_expanding;	/* expand the vdev?		*/
 	boolean_t	vdev_reopening;	/* reopen in progress?		*/
+	boolean_t	vdev_nonrot;	/* true if solid state		*/
 	int		vdev_open_error; /* error on last open		*/
 	kthread_t	*vdev_open_thread; /* thread opening children	*/
 	uint64_t	vdev_crtxg;	/* txg when top-level was added */