libspl/random: add switch to force pseudo-random numbers for all calls

ztest wants to force all kernel random calls to use the pseudo-random
generator (/dev/urandom), to avoid depleting the system entropy pool
just for testing.

Up until the previous commit, it did this by switching the path that the
libzpool (now libspl) random API would use to get random data from; that
is, it took advantage of an implementation detail.

Now that that hole is closed to it, we need another method. This commit
introduces that; a simple API call to enable/disable "force pseudo"
mode.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17861

cmd/ztest.c

@@ -8952,6 +8952,12 @@ main(int argc, char **argv)
 
 	libspl_init();
 
+	/*
+	 * Force random_get_bytes() to use /dev/urandom in order to prevent
+	 * ztest from needlessly depleting the system entropy pool.
+	 */
+	random_force_pseudo(B_TRUE);
+
 	if (!fd_data_str) {
 		process_options(argc, argv);