swiotlb: don't assume PA 0 is invalid

In 2.6.29 io_tlb_orig_addr[] got converted from storing virtual addresses
to storing physical ones. While checking virtual addresses against NULL
is a legitimate thing to catch invalid entries, checking physical ones
against zero isn't: There's no guarantee that PFN 0 is reserved on a
particular platform.

Since it is unclear whether the check in swiotlb_tbl_unmap_single() is
actually needed, retain it but check against a guaranteed invalid physical
address. This requires setting up the array in a suitable fashion. And
since the original code failed to invalidate array entries when regions
get unmapped, this is being fixed at once along with adding a similar
check to swiotlb_tbl_sync_single().

Obviously the less intrusive change would be to simply drop the check in
swiotlb_tbl_unmap_single().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Merge tag 'stable/for-linus-3.16-rc1-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/konrad/swiotlb

Pull swiotlb bugfix from Konrad Rzeszutek Wilk:
 "One bug-fix that had been in tree for quite some time.  We had assumed
  that the physical address zero was invalid and would fail it.  But
  that is not true and on some architectures it is not reserved and
  valid.  This fixes it"

* tag 'stable/for-linus-3.16-rc1-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/konrad/swiotlb:
  swiotlb: don't assume PA 0 is invalid