ip4: calculate ip_capacity without knowing ethernet

Author: alfreb

api/net/ip4/packet_ip4.hpp

@@ -57,9 +57,8 @@ namespace net {
     uint16_t ip_data_length() const noexcept
     { return ip_segment_length() - ip_header_length(); }
 
-    // TODO: Remove Ethernet dependency, some day...
     uint16_t ip_capacity() const noexcept
-    { return capacity() - ip_full_header_length() - sizeof(LinkLayer::trailer); }
+    { return capacity() - ip_header_length(); }
 
     void set_ip_data_length(uint16_t length) {
       Expects(data_end() >= layer_begin());

api/net/packet.hpp

@@ -20,6 +20,7 @@
 
 #include "buffer_store.hpp"
 #include "ip4/addr.hpp"
+#include <gsl/gsl_assert>
 #include <delegate>
 #include <cassert>