netfilter: xtables: hardidletimer target implementation

This patch implements a hardidletimer Xtables target that can be used to
identify when interfaces have been idle for a certain period of time.

Timers are identified by labels and are created when a rule is set with a
new label. The rules also take a timeout value (in seconds) as an option.
If more than one rule uses the same timer label, the timer will be
restarted whenever any of the rules get a hit.

One entry for each timer is created in sysfs. This attribute contains the
timer remaining for the timer to expire. The attributes are located under
the xt_idletimer class:

/sys/class/xt_hardidletimer/timers/<label>

When the timer expires, the target module sends a sysfs notification to the
userspace, which can then decide what to do (eg. disconnect to save power)

Compared to xt_IDLETIMER, xt_HARDIDLETIMER can send notifications when CPU
is in suspend too, to notify the timer expiry

Change-Id: I1723d44e02df864d63da7550766ea7d763076de2
Signed-off-by: Bhavya Sokke Mallikarjunappa <bsokke@codeaurora.org>