rxrpc: Don't store the rxrpc header in the Tx queue sk_buffs

Don't store the rxrpc protocol header in sk_buffs on the transmit queue,
but rather generate it on the fly and pass it to kernel_sendmsg() as a
separate iov.  This reduces the amount of storage required.

Note that the security header is still stored in the sk_buff as it may get
encrypted along with the data (and doesn't change with each transmission).

Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/conn_object.c

@@ -53,7 +53,6 @@ struct rxrpc_connection *rxrpc_alloc_connection(gfp_t gfp)
 		spin_lock_init(&conn->state_lock);
 		conn->debug_id = atomic_inc_return(&rxrpc_debug_id);
 		conn->size_align = 4;
-		conn->header_size = sizeof(struct rxrpc_wire_header);
 		conn->idle_timestamp = jiffies;
 	}