Wait interruptibly in prefetch thread

Wait interruptibly in prefetch thread

The Linux kernel watchdog will automatically dump a backtrace for
any process while sleeps for over 120s in an uninterruptible state.

The solution is for the prefetch thread to sleep in an interruptible
state.  The way the existing code was written this is safe because
when woken it will always reevaluate its conditional.  As a general
rule it is preferable to sleep in an interruptible when possible.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3450
Closes #3402