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>