Optimize small random numbers generation

In all places except two spa_get_random() is used for small values,
and the consumers do not require well seeded high quality values.
Switch those two exceptions directly to random_get_pseudo_bytes()
and optimize spa_get_random(), renaming it to random_in_range(),
since it is not related to SPA or ZFS in general.

On FreeBSD directly map random_in_range() to new prng32_bounded() KPI
added in FreeBSD 13.  On Linux and in user-space just reduce the type
used to uint32_t to avoid more expensive 64bit division.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12183

module/zfs/vdev_mirror.c

@@ -496,7 +496,7 @@ vdev_mirror_preferred_child_randomize(zio_t *zio)
 	int p;
 
 	if (mm->mm_root) {
-		p = spa_get_random(mm->mm_preferred_cnt);
+		p = random_in_range(mm->mm_preferred_cnt);
 		return (vdev_mirror_dva_select(zio, p));
 	}