mptcp: fix subflow rcvbuf adjust

The mptcp PM can add subflow to the conn_list before tcp_init_transfer().
Calling tcp_rcvbuf_grow() on such subflow is not correct as later
init will overwrite the update.

Fix the issue calling tcp_rcvbuf_grow() only after init buffer
initialization.

Fixes: e118cdc ("mptcp: rcvbuf auto-tuning improvement")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251028-net-tcp-recv-autotune-v3-1-74b43ba4c84c@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/protocol.c

@@ -2051,6 +2051,7 @@ static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
 
 	msk->rcvq_space.space = msk->rcvq_space.copied;
 	if (mptcp_rcvbuf_grow(sk)) {
+		int copied = msk->rcvq_space.copied;
 
 		/* Make subflows follow along.  If we do not do this, we
 		 * get drops at subflow level if skbs can't be moved to
@@ -2063,8 +2064,11 @@ static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
 
 			ssk = mptcp_subflow_tcp_sock(subflow);
 			slow = lock_sock_fast(ssk);
-			tcp_sk(ssk)->rcvq_space.space = msk->rcvq_space.copied;
-			tcp_rcvbuf_grow(ssk);
+			/* subflows can be added before tcp_init_transfer() */
+			if (tcp_sk(ssk)->rcvq_space.space) {
+				tcp_sk(ssk)->rcvq_space.space = copied;
+				tcp_rcvbuf_grow(ssk);
+			}
 			unlock_sock_fast(ssk, slow);
 		}
 	}