Simplify and optimize random_int_between().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes openzfs#14805

tests/zfs-tests/include/math.shlib

@@ -118,9 +118,7 @@ function verify_ne # <a> <b> <type>
 
 # A simple function to get a random number between two bounds (inclusive)
 #
-# Probably not the most efficient for large ranges, but it's okay.
-#
-# Note since we're using $RANDOM, 32767 is the largest number we
+# Note since we're using $RANDOM, $min+32767 is the largest number we
 # can accept as the upper bound.
 #
 # $1 lower bound
@@ -129,11 +127,6 @@ function random_int_between
 {
 	typeset -i min=$1
 	typeset -i max=$2
-	typeset -i rand=0
-
-	while [[ $rand -lt $min ]] ; do
-		rand=$(( $RANDOM % $max + 1))
-	done
 
-	echo $rand
+	echo $(( (RANDOM % (max - min + 1)) + min ))
 }