block: Prevent hang_check firing during long I/O

During long I/O operations, the hang_check timer may fire,
trigger stack dumps that unnecessarily alarm the user.

Eg.  hdparm --security-erase NULL /dev/sdb  ## can take *hours* to complete

So, if hang_check is armed, we should wake up periodically
to prevent it from triggering.  This patch uses a wake-up interval
equal to half the hang_check timer period, which keeps overhead low enough.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>