igbv: Fence DMA while sanitizing retained queuesHEADmain

The 82576 and I350 retain VF queue enable and DMA address state
across VFLR.  iflib enables PCI bus mastering before driver attach, so
stale state left by a previous owner can otherwise issue DMA before
igbvf has completed its first reset and queue sanitization.

Disable PCI bus mastering immediately after mapping the VF BAR.  Keep
it disabled until reset and queue sanitization succeed, verify both
disable and enable through PCI command-register readback, and wait for
pending transactions before treating the fence as complete.
Resanitize on stop before iflib releases queue mappings.

The sanitizer and recovery were exercised on I350 and 82576 VFs.  Forced
queue-disable failure left the VF down, and a later administrative
down/up recovered it; successful I350 VFs passed bidirectional traffic
with no errors or drops.

Sponsored by:	BBOX.io

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