selftests: mptcp: pm: restrict 'unknown' check to pm_nl_ctl

selftests: mptcp: pm: restrict 'unknown' check to pm_nl_ctl

When pm_netlink.sh is executed with '-i', 'ip mptcp' is used instead of
'pm_nl_ctl'. IPRoute2 doesn't support the 'unknown' flag, which has only
been added to 'pm_nl_ctl' for this specific check: to ensure that the
kernel ignores such unsupported flag.

No reason to add this flag to 'ip mptcp'. Then, this check should be
skipped when 'ip mptcp' is used.

Fixes: 0cef6fcac24d ("selftests: mptcp: ip_mptcp option for more scripts")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260505-net-mptcp-pm-fixes-7-1-rc3-v1-11-fca8091060a4@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>