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

include/os/freebsd/spl/sys/random.h

@@ -30,6 +30,9 @@
 #define	_OPENSOLARIS_SYS_RANDOM_H_
 
 #include_next <sys/random.h>
+#if  __FreeBSD_version >= 1300108
+#include <sys/prng.h>
+#endif
 
 static inline int
 random_get_bytes(uint8_t *p, size_t s)
@@ -45,4 +48,23 @@ random_get_pseudo_bytes(uint8_t *p, size_t s)
 	return (0);
 }
 
+static inline uint32_t
+random_in_range(uint32_t range)
+{
+#if  __FreeBSD_version >= 1300108
+	return (prng32_bounded(range));
+#else
+	uint32_t r;
+
+	ASSERT(range != 0);
+
+	if (range == 1)
+		return (0);
+
+	(void) random_get_pseudo_bytes((void *)&r, sizeof (r));
+
+	return (r % range);
+#endif
+}
+
 #endif	/* !_OPENSOLARIS_SYS_RANDOM_H_ */

include/os/linux/spl/sys/random.h

@@ -36,4 +36,19 @@ random_get_bytes(uint8_t *ptr, size_t len)
 
 extern int random_get_pseudo_bytes(uint8_t *ptr, size_t len);
 
+static __inline__ uint32_t
+random_in_range(uint32_t range)
+{
+	uint32_t r;
+
+	ASSERT(range != 0);
+
+	if (range == 1)
+		return (0);
+
+	(void) random_get_pseudo_bytes((void *)&r, sizeof (r));
+
+	return (r % range);
+}
+
 #endif	/* _SPL_RANDOM_H */