mptcp: pm nl: reduce variable scope

To avoid confusions like when working on the previous patch, better to
declare and assign this variable only where it is needed.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>