Fix out-of-bound in per_cpu in spl_random_init

When iterating per_cpu values, we need to use for_each_possible_cpu. While
NR_CPUS indicates the number of CPU supported by the kernel, it might not
initialize all of them if the kernel decides it's not possible to use them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #578

module/spl/spl-generic.c

@@ -638,7 +638,7 @@ spl_random_init(void)
 		    "0x%016llx%016llx.", cpu_to_be64(s[0]), cpu_to_be64(s[1]));
 	}
 
-	for (i = 0; i < NR_CPUS; i++) {
+	for_each_possible_cpu(i) {
 		uint64_t *wordp = per_cpu(spl_pseudo_entropy, i);
 
 		spl_rand_jump(s);