mptcp: remove tcp ulp setsockopt support

In the Linux kernel, the following vulnerability has been resolved:

mptcp: remove tcp ulp setsockopt support

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.