IOMMU/x86: avoid pages without GFN in page table creation/updating

IOMMU/x86: avoid pages without GFN in page table creation/updating

Handing INVALID_GFN to functions like hd->platform_ops->map_page()
just can't do any good, and the ioreq server code results in such pages
being on the list of ones owned by a guest.

While - as suggested by Tim - we should use get_gfn()/put_gfn() there
to eliminate races, we really can't due to holding the domain's page
alloc lock. Ultimately arch_iommu_populate_page_table() may need to be
switched to be GFN based. Here is what Tim said in this regard:
"Ideally this loop would be iterating over all gfns in the p2m rather
 than over all owned MFNs.  As long as needs_iommu gets set first,
 such a loop could safely be paused and restarted without worrying
 about concurrent updates.  The code sould even stay in this file,
 though exposing an iterator from the p2m code would be a lot more
 efficient."

Original by Andrew Cooper <andrew.cooper3@citrix.com>, using further
suggestions from Tim Deegan <tim@xen.org>.

Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Yang Zhang <yang.z.zhang@intel.com>
Acked-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>