mptcp: allocate TX skbs in msk context

Move the TX skbs allocation in mptcp_sendmsg() scope,
and tentatively pre-allocate a skbs number proportional
to the sendmsg() length.

Use the ssk tx skb cache to prevent the subflow allocation.

This allows removing the msk skb extension cache and will
make possible the later patches.
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.h

@@ -240,11 +240,13 @@ struct mptcp_sock {
 	struct sk_buff  *ooo_last_skb;
 	struct rb_root  out_of_order_queue;
 	struct sk_buff_head receive_queue;
+	struct sk_buff_head skb_tx_cache;	/* this is wmem accounted */
+	int		tx_pending_data;
+	int		size_goal_cache;
 	struct list_head conn_list;
 	struct list_head rtx_queue;
 	struct mptcp_data_frag *first_pending;
 	struct list_head join_list;
-	struct skb_ext	*cached_ext;	/* for the next sendmsg */
 	struct socket	*subflow; /* outgoing connect/listener/!mp_capable */
 	struct sock	*first;
 	struct mptcp_pm_data	pm;