rfc4861: encode: fix reserved field size

Fix reserved field size when encoding packet:

https://tools.ietf.org/html/rfc4861#section-4.1

Reported in:

msantos#44

Thanks @alexshavelev!

src/pkt_icmp6.erl

@@ -1,4 +1,4 @@
-%% Copyright (c) 2009-2015, Michael Santos <[email protected]>
+%% Copyright (c) 2009-2017, Michael Santos <[email protected]>
 %% All rights reserved.
 %%
 %% Redistribution and use in source and binary forms, with or without
@@ -96,7 +96,7 @@ codec(#icmp6{
     <<Type:8, Code:8, Checksum:16, Id:16, Seq:16>>;
 
 %%
-%% RFC 4681: Neighbor Discovery for IP version 6 (IPv6)
+%% RFC 4861: Neighbor Discovery for IP version 6 (IPv6)
 %%
 
 % Router Solicitation Message
@@ -109,7 +109,7 @@ codec(#icmp6{
         type = ?ND_ROUTER_SOLICIT, code = Code, checksum = Checksum,
         res = Res
     }) ->
-    <<?ND_ROUTER_SOLICIT:8, Code:8, Checksum:16, Res:16>>;
+    <<?ND_ROUTER_SOLICIT:8, Code:8, Checksum:16, Res:32>>;
 
 % Router Advertisement Message
 codec(<<?ND_ROUTER_ADVERT:8, Code:8, Checksum:16, Hop:8, M:1, O:1, Res:6,