selftests: mptcp: try to set mptcp ulp mode in different sk states

The kernel will crash without
'mptcp: clear 'kern' flag from fallback sockets' change.

Since this doesn't slow down testing in a noticeable way,
run this unconditionally.

The explicit test did not catch this, because the check was done
for tcp socket returned by 'socket(.. IPPROTO_TCP) rather than a
tcp socket returned by accept() on a mptcp listen fd.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>