xen: increase __PHYSICAL_MASK_SHIFT_XEN to 52

The current value of __PHYSICAL_MASK_SHIFT_XEN in crash (40) is
smaller than the kernel (52) since kernel commit 6f0e8bf167 (xen:
support 52 bit physical addresses in pv guests).

This can cause x86_64_pud_offset() to lose the most significant
bits of pgd_pte, leading to a failed xen_m2p() translation,
resulting in crash failing with an error message like this:
   crash: read error: physical address: ffffffffffffffff  type: "pud page"

Both Intel and AMD documentation mandate that unused physical
address bits must be 0, so there is no need to explicitly mask them
out with a mask narrower than the architecture limit of 52. This
is also confirmed by this kernel commit: b83ce5ee91.

Increase the value of __PHYSICAL_MASK_SHIFT_XEN to 52.

Signed-off-by: Jiri Bohac <jbohac@suse.cz>

defs.h

@@ -3585,7 +3585,7 @@ struct arm64_stackframe {
  *  PHYSICAL_PAGE_MASK changed (enlarged) between 2.4 and 2.6, so
  *  for safety, use the 2.6 values to generate it.
  */ 
-#define __PHYSICAL_MASK_SHIFT_XEN     40
+#define __PHYSICAL_MASK_SHIFT_XEN     52
 #define __PHYSICAL_MASK_SHIFT_2_6     46
 #define __PHYSICAL_MASK_SHIFT_5LEVEL  52
 #define __PHYSICAL_MASK_SHIFT  (machdep->machspec->physical_mask_shift)