mptcp: add SNDTIMEO setsockopt support

Add setsockopt support for SO_SNDTIMEO_OLD and SO_SNDTIMEO_NEW to fix this
error reported by the mptcp bpf selftest:

 (network_helpers.c:64: errno: Operation not supported) Failed to set SO_SNDTIMEO
 test_mptcp:FAIL:115

 All error logs:

 (network_helpers.c:64: errno: Operation not supported) Failed to set SO_SNDTIMEO
 test_mptcp:FAIL:115
 Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/sockopt.c

@@ -343,6 +343,8 @@ static int mptcp_setsockopt_sol_socket(struct mptcp_sock *msk, int optname,
 	case SO_RCVLOWAT:
 	case SO_RCVTIMEO_OLD:
 	case SO_RCVTIMEO_NEW:
+	case SO_SNDTIMEO_OLD:
+	case SO_SNDTIMEO_NEW:
 	case SO_BUSY_POLL:
 	case SO_PREFER_BUSY_POLL:
 	case SO_BUSY_POLL_BUDGET: