…D#64):

	sys/dev/pci/pvscsi.c: revision 1.4
	sys/dev/pci/pvscsi.c: revision 1.5

pvscsi(4): Use bus_dmamap_sync, not membar_*, for DMA.
membar_* may be a noop if we're booting on a single _virtual_ CPU,
but the barriers are still needed in case the host behind pvscsi(4)
is running on another _physical_ CPU.

Prompted by (and related to but not the same issue as):
PR kern/59618: occasional virtio block device lock ups/hangs


pvscsi(4): Zero rings before using them.

bus_dmamem_alloc(9) doesn't guarantee zeroing, as far as I can tell,
and who knows what might happen if the ring header and contents have
anything nonzero initially.

Insert an initial preread/prewrite sync between zeroing and first
use, and, out of paranoia, a final postread/postwrite sync between
last use and unload/free.

Prompted by (but not really related to):
PR kern/59618: occasional virtio block device lock ups/hangs