Clarified packet ownership transfer between a few functions.

Added a large number of missing calls to free_pkb().  In the case of UDP,
no received packets were ever freed, which lead to memory exhaustion
remarkably quickly once pxelinux started up.

In general, any function with _rx() in its name which accepts a pk_buff
*must* either call free_pkb() or pass the pkb to another _rx() function
(e.g. the next layer up the stack).  Since the UDP (and TCP) layers don't
pass packet buffers up to the higher-layer protocols (the
"applications"), they must free the packet buffer after calling the
application's newdata() method.

src/net/tcpip.c

@@ -54,6 +54,7 @@ int tcpip_rx ( struct pk_buff *pkb, uint8_t tcpip_proto,
 	}
 
 	DBG ( "Unrecognised TCP/IP protocol %d\n", tcpip_proto );
+	free_pkb ( pkb );
 	return -EPROTONOSUPPORT;
 }
 
@@ -78,6 +79,7 @@ int tcpip_tx ( struct pk_buff *pkb, struct tcpip_protocol *tcpip_protocol,
 	}
 	
 	DBG ( "Unrecognised TCP/IP address family %d\n", st_dest->st_family );
+	free_pkb ( pkb );
 	return -EAFNOSUPPORT;
 }