network/routing-policy-rule: use int32_t for suppress_prefixlen

The kernel parses FRA_SUPPRESS_PREFIXLEN as uint32_t, but internally
handled as signed integer and negative values as unset. Let's explicitly
specify the size of the variable.

No functional change, just refactoring.

src/basic/parse-util.c

@@ -714,7 +714,7 @@ int parse_ip_port_range(const char *s, uint16_t *low, uint16_t *high, bool allow
         return 0;
 }
 
-int parse_ip_prefix_length(const char *s, int *ret) {
+int parse_ip_prefix_length(const char *s, int32_t *ret) {
         unsigned l;
         int r;
 
@@ -725,7 +725,7 @@ int parse_ip_prefix_length(const char *s, int *ret) {
         if (l > 128)
                 return -ERANGE;
 
-        *ret = (int) l;
+        *ret = (int32_t) l;
 
         return 0;
 }