unconditionally stop other cpus when entering kdb in smp system
unconditionally stop other cpus when entering kdb in smp system

... and thus retire debug.kdb.stop_cpus tunable/sysctl.
The knob was to work around CPU stopping issues, which since have been
either fixed or greatly reduced.  kdb should really operate in a special
environment with scheduler stopped and interrupts disabled to provide
deterministic debugging.

Discussed with:	attilio, rwatson
X-MFC after:	2 months or never

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