mptcp: reduce branching when writing MP_FAIL option

MP_FAIL should be use in very rare cases, either when the TCP RST flag
is set -- with or without an MP_RST -- or with a DSS, see
mptcp_established_options().

Here, we do the same in mptcp_write_options().

Co-developed-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>