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.
Note that this parameter is ignored when rcutorture is a module rather
than built into the kernel.  This could be changed with the addition of
an EXPORT_SYMBOL_GPL().

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>