mptcp: pm: netlink: fix if-idx type

mptcp: pm: netlink: fix if-idx type

As pointed out by Donald, when parsing an entry, the wrong type was set
for the temp value: this value is signed.

There are no real issues here, because the intermediate variable was
only wrong for the sign, not for the size, and the final variable had
the right sign. But this feels wrong, and is confusing, so fixing this
small typo introduced by commit ef0da3b8a2f1 ("mptcp: move address
attribute into mptcp_addr_info").

Reported-by: Donald Hunter <donald.hunter@gmail.com>
Closes: https://lore.kernel.org/m2plc0ui9z.fsf@gmail.com
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-next-mptcp-minor-fixes-6-18-v1-3-99d179b483ad@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>