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

module/zfs/vdev_file.c

@@ -57,6 +57,9 @@ vdev_file_open(vdev_t *vd, uint64_t *psize, uint64_t *max_psize,
 	vattr_t vattr;
 	int error;
 
+	/* Rotational optimizations only make sense on block devices */
+	vd->vdev_nonrot = B_TRUE;
+
 	/*
 	 * We must have a pathname, and it must be absolute.
 	 */