[PATCH mptcp] mptcp: remove tcp ulp setsockopt support

[Florian Westphal]

TCP_ULP setsockopt cannot be used for mptcp because its already
used internally to plumb subflow (tcp) sockets to the mptcp layer.

syzbot managed to trigger a crash for mptcp connections that are
in fallback mode:

KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
CPU: 1 PID: 1083 Comm: syz-executor.3 Not tainted 5.16.0-rc2-syzkaller #0
RIP: 0010:tls_build_proto net/tls/tls_main.c:776 [inline]
[..]
 __tcp_set_ulp net/ipv4/tcp_ulp.c:139 [inline]
 tcp_set_ulp+0x428/0x4c0 net/ipv4/tcp_ulp.c:160
 do_tcp_setsockopt+0x455/0x37c0 net/ipv4/tcp.c:3391
 mptcp_setsockopt+0x1b47/0x2400 net/mptcp/sockopt.c:638

Remove support for TCP_ULP setsockopt.

Reported-by: syzbot+1fd9b69cde42967d1add@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 3c3db22fd36a..aa3fcd86dbe2 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -543,7 +543,6 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		case TCP_NODELAY:
 		case TCP_THIN_LINEAR_TIMEOUTS:
 		case TCP_CONGESTION:
-		case TCP_ULP:
 		case TCP_CORK:
 		case TCP_KEEPIDLE:
 		case TCP_KEEPINTVL:
-- 
2.32.0