mptcp: pm: drop inet6_sk after inet_sk

mptcp: pm: drop inet6_sk after inet_sk

In mptcp_event_add_subflow(), mptcp_event_pm_listener() and
mptcp_nl_find_ssk(), 'issk' has already been got through inet_sk().

No need to use inet6_sk() to get 'ipv6_pinfo' again, just use
issk->pinet6 instead. This patch also drops these 'ipv6_pinfo'
variables.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250221-net-next-mptcp-pm-misc-cleanup-3-v1-6-2b70ab1cee79@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>