UPSTREAM: netfilter: xt_IDLETIMER: fix sysfs callback function type

Use struct device_attribute instead of struct idletimer_tg_attr, and
the correct callback function type to avoid indirect call mismatches
with Control Flow Integrity checking.

Bug: 117299373
Change-Id: I5cc50171e01a5162ef504a726fe3ed4b51941b34
(cherry-picked from commit 20fdaf6e1e313b4efa48243d5250526eb43ea70d)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>