IOMMU/x86: fix type ref-counting race upon IOMMU page table construction

IOMMU/x86: fix type ref-counting race upon IOMMU page table construction

When arch_iommu_populate_page_table() gets invoked for an already
running guest, simply looking at page types once isn't enough, as they
may change at any time. Add logic to re-check the type after having
mapped the page, unmapping it again if needed.

This is XSA-285.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tentatively-Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 1f0b0bb7773d537bcf169e021495d0986d9809fc
master date: 2019-03-05 13:47:36 +0100