netfilter: xt_HARDIDLETIMER: Fix use after free condition

Force any pending hardidletimer_tg_work() to complete before freeing
the associated work struct.

CRs-Fixed: 814707
Change-Id: I57b2f0dcd24f05ddb472d6007525d1722f9fe0b0
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>