rcutorture: Add test_nmis module parameter

This commit adds a test_nmis module parameter to generate the specified
number of NMI stack backtraces 15 seconds apart.  This module parameter
can be used to test NMI delivery and accompanying diagnostics.

It also forces NMI backtraces in the case where the destination CPU is
idle, thus avoiding the occasional annoying failure to actually send
an NMI.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2 files changed