With DIAGNOSTICS, sometimes we get weird crashes when some driver accesses softc after it is freed.  Use a different malloc type for softc than the rest of the bus code to make it more clear when these things happen that it is the driver that's at fault, not the bus code.
With DIAGNOSTICS, sometimes we get weird crashes when some driver
accesses softc after it is freed.  Use a different malloc type for
softc than the rest of the bus code to make it more clear when these
things happen that it is the driver that's at fault, not the bus code.

Suggested by: sam and/or phk (I think)

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