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/multilist.c

@@ -20,9 +20,6 @@
 #include <sys/multilist.h>
 #include <sys/trace_zfs.h>
 
-/* needed for spa_get_random() */
-#include <sys/spa.h>
-
 /*
  * This overrides the number of sublists in each multilist_t, which defaults
  * to the number of CPUs in the system (see multilist_create()).
@@ -275,7 +272,7 @@ multilist_get_num_sublists(multilist_t *ml)
 unsigned int
 multilist_get_random_index(multilist_t *ml)
 {
-	return (spa_get_random(ml->ml_num_sublists));
+	return (random_in_range(ml->ml_num_sublists));
 }
 
 /* Lock and return the sublist specified at the given index */