Drop kernel lock before panic to avoid WITNESS report during fault

Drop kernel lock before panic to avoid WITNESS report during fault

holding a spinlock, eg. malloc's malloc_mutex in "Data modified on freelist ..."
triggers "acquiring blockable sleep lock with spinlock or critical section held"
since kpageflttrap() grabs the kernel lock before fault() to serialise multiple
threds/faults avoid interleaved console text.

But fault() immediately sets the per-CPU panic string, so the kernel lock does
not really help here.

Use 'show panic' to recover from garbled console text if need be, as usual.
The i386 equivalent does not use the kernel lock, either.

OK bluhm kettenis