x86: skip check for spurious faults for non-present faults

If a fault on a kernel address is due to a non-present page, then it
cannot be the result of stale TLB entry from a protection change (RO
to RW or NX to X).  Thus the pagetable walk in spurious_fault() can be
skipped.

See the initial if in spurious_fault() and the tests in
spurious_fault_check()) for the set of possible error codes checked
for spurious faults.  These are:

         IRUWP
Before   x00xx && ( 1xxxx || xxx1x )
After  ( 10001 || 00011 ) && ( 1xxxx || xxx1x )

Thus the new condition is a subset of the previous one, excluding only
non-present faults (I == 1 and W == 1 are mutually exclusive).

This avoids spurious_fault() oopsing in some cases if the pagetables
it attempts to walk are not accessible.  This obscures the location of
the original fault.

This also fixes a crash with Xen PV guests when they access entries in
the M2P corresponding to device MMIO regions.  The M2P is mapped
(read-only) by Xen into the kernel address space of the guest and this
mapping may contains holes for non-RAM regions.  Read faults will
result in calls to spurious_fault(), but because the page tables for
the M2P mappings are not accessible by the guest the pagetable walk
would fault.

This was not normally a problem as MMIO mappings would not normally
result in a M2P lookup because of the use of the _PAGE_IOMAP bit the
PTE.  However, removing the _PAGE_IOMAP bit requires M2P lookups for
MMIO mappings as well.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>

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