Land rapid7#4102, @jhart-r7's fix for nbns_response

* Use request src_port instead of 137

Author: jvazquez-r7

modules/auxiliary/spoof/nbns/nbns_response.rb

@@ -67,6 +67,7 @@ def run
 
     while @run # Not exactly thrilled we can never turn this off XXX fix this sometime.
       packet, addr = @sock.recvfrom(512)
+      src_port = addr[1]
       rhost = addr[3]
 
       break if packet.length == 0
@@ -127,7 +128,7 @@ def run
         p.ip_daddr = rhost
         p.ip_ttl = 255
         p.udp_sport = 137
-        p.udp_dport = 137
+        p.udp_dport = src_port
         p.payload = response
         p.recalc