lib: Add backtrace_idle parameter to force backtrace of idle CPUs

lib: Add backtrace_idle parameter to force backtrace of idle CPUs

Currently, the nmi_cpu_backtrace() declines to produce backtraces for
idle CPUs.  This is a good choice in the common case in which problems are
caused only by non-idle CPUs.  However, there are occasionally situations
in which idle CPUs are helping to cause problems.  This commit therefore
adds an nmi_backtrace.backtrace_idle kernel boot parameter that causes
nmi_cpu_backtrace() to dump stacks even of idle CPUs.

Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: <[email protected]>