Convert netisr to use dynamic per-CPU storage (DPCPU) instead of sizing arrays to [MAXCPU], offering moderate memory savings.  In some places, this requires using CPU_ABSENT() to handle less common platforms with sparse CPU IDs.  In several places, assert that the selected CPUID for work placement or statistics is not CPU_ABSENT() to be on the safe side.
Convert netisr to use dynamic per-CPU storage (DPCPU) instead of sizing
arrays to [MAXCPU], offering moderate memory savings.  In some places,
this requires using CPU_ABSENT() to handle less common platforms with
sparse CPU IDs.  In several places, assert that the selected CPUID for
work placement or statistics is not CPU_ABSENT() to be on the safe side.

Discussed with:	bz, jeff

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