this avoids a strange hang at reboot i am seeing on an old pentium4-m
laptop, that was introduced with kern_mutex.c 1.91/92, though i can
not really explain why that matters (in the waiting thread, a pointer
that should be NULL remains non NULL.) thanks to jmcneill@ for some
helpful review commentary here.
don't panic() if either "cardbus" or "pcmcia" didn't attach and a
card is inserted. this can happen for various reasons, including
some regression in netbsd (-current, and -9, at least) that suggests
using PCI_BUS_FIXUP (though it still fails to attach the card i have.)
both found with GCC 10 testing, though both also occur with GCC 7 in
the netbsd-9 tree as well.
856324edf9