kernel/watchdog.c: add sysctl knob hardlockup_panic

kernel/watchdog.c: add sysctl knob hardlockup_panic

The only way to enable a hardlockup to panic the machine is to set
'nmi_watchdog=panic' on the kernel command line.

This makes it awkward for end users and folks who want to run automate
tests (like myself).

Mimic the softlockup_panic knob and create a /proc/sys/kernel/hardlockup_panic
knob.

Signed-off-by: Don Zickus <[email protected]>
Cc: Ulrich Obergfell <[email protected]>
Acked-by: Jiri Kosina <[email protected]>
Reviewed-by: Aaron Tomlin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>