mptcp: define more local variables sk

mptcp: define more local variables sk

'(struct sock *)msk' is used several times in mptcp_nl_cmd_announce(),
mptcp_nl_cmd_remove() or mptcp_userspace_pm_set_flags() in pm_userspace.c,
it's worth adding a local variable sk to point it.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-8-db8f25f798eb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>