This eliminates problems where the underlying interrupt handler isn't the
specific layer calling scsipi_complete() for a given scsi transaction.
This avoids deadlocks where the kthread that called the autoconf routines
to configure a scsibus shouldn't be the one put to sleep waiting on a
scsipi_complete (only the scsibus's kthread should be doing that).
To avoid jitter this will force the scsibus's to probe in the order they
run through autoconf (so machines with multiple bus's don't move sd* devices
around on every reboot).
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