Fix unaligned memory access when creating ICMP Port Unreachable messages

commit 3b69d09c80 corrected the
unreachable handling for net/udp/icmp but introduced an unaligned store.
This splits the uint32_t data field into a two element uint16_t data
field to avoid the unaligned store.

include/nuttx/net/icmp.h

@@ -141,7 +141,7 @@ struct icmp_hdr_s
           uint16_t seqno;   /* "  " "" "   " "      " "  " "     " */
         };
 
-      uint32_t data;
+      uint16_t data[2];
     };
 };
 

include/nuttx/net/icmpv6.h

@@ -157,7 +157,7 @@ struct icmpv6_hdr_s
    * message type indicated by the Type and Code fields.
    */
 
-  uint32_t data;
+  uint16_t data[2];
 };
 
 /* The ICMPv6 and IPv6 headers */

net/icmp/icmp_reply.c

@@ -153,7 +153,8 @@ void icmp_reply(FAR struct net_driver_s *dev, int type, int code)
 
   icmp->type        = type;
   icmp->icode       = code;
-  icmp->data        = 0;
+  icmp->data[0]     = 0;
+  icmp->data[1]     = 0;
 
   /* Calculate the ICMP checksum. */
 

net/icmpv6/icmpv6_reply.c

@@ -133,9 +133,10 @@ void icmpv6_reply(FAR struct net_driver_s *dev, int type, int code, int data)
 
   /* Initialize the ICMPv6 header */
 
-  icmpv6->type   = type;
-  icmpv6->code   = code;
-  icmpv6->data   = htonl(data);
+  icmpv6->type    = type;
+  icmpv6->code    = code;
+  icmpv6->data[0] = data >> 16;
+  icmpv6->data[1] = data & 0xffff;
 
   /* Calculate the ICMPv6 checksum over the ICMPv6 header and payload. */