kern_fail: Allow sleeping for more than 2147483/hz seconds

Because of integer types, the timeout calculation result was limited to
INT_MAX / (1000 * hz) seconds. For systems with hz=10000, this is only 215
seconds. Perform the calculation with 64-bit math to allow sleeping for the
full INT_MAX / hz interval (215000 seconds on such hz=10000 systems).

Submitted by: Scott Ferris <sferris at isilon.com>
Sponsored by: Dell EMC Isilon