bus/pci: forbid IOVA mode if IOMMU address width too small

bus/pci: forbid IOVA mode if IOMMU address width too small

Intel VT-d supports different address widths for the IOVAs, from
39 bits to 56 bits.

While recent processors support at least 48 bits, VT-d emulation
currently only supports 39 bits. It makes DMA mapping to fail in this
case when using VA as IOVA mode, as user-space virtual addresses uses
up to 47 bits (see kernel's Documentation/x86/x86_64/mm.txt).

This patch parses VT-d CAP register value available in sysfs, and
forbid VA as IOVA mode if the GAW is 39 bits or unknown.

Fixes: f37dfab21c98 ("drivers/net: enable IOVA mode for Intel PMDs")
Cc: stable@dpdk.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Chas Williams <chas3@att.com>