uma: Add KMSAN hooks

uma: Add KMSAN hooks

For now, just hook the allocation path: upon allocation, items are
marked as initialized (absent M_ZERO).  Some zones are exempted from
this when it would otherwise raise false positives.

Use kmsan_orig() to update the origin map for UMA and malloc(9)
allocations.  This allows KMSAN to print the return address when an
uninitialized UMA item is implicated in a report.  For example:
  panic: MSan: Uninitialized UMA memory from m_getm2+0x7fe

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