checkpolicy: Check the right bits of an ibpkeycon rule subnet prefix

The lower 64 bits of the subnet prefix for an ibpkeycon rule should
all be 0's. Unfortunately the check uses the s6_addr macro which refers
to the 16 entry array of 8-bit values in the union and does not refer
to the correct bits.

Use the s6_addr32 macro instead which refers to the 4 entry array of
32-bit values in the union and refers to the lower 64 bits.

Signed-off-by: James Carter <jwcart2@gmail.com>

checkpolicy/policy_define.c

@@ -5036,7 +5036,7 @@ int define_ibpkey_context(unsigned int low, unsigned int high)
 		goto out;
 	}
 
-	if (subnet_prefix.s6_addr[2] || subnet_prefix.s6_addr[3]) {
+	if (subnet_prefix.s6_addr32[2] || subnet_prefix.s6_addr32[3]) {
 		yyerror("subnet prefix should be 0's in the low order 64 bits.");
 		rc = -1;
 		goto out;