pdx: cast PAGE_SIZE value ahead of shifting

Coverity complains that:

 In expression 0x1000 << bottom_shift, left shifting by more than 31 bits
 has undefined behavior. The shift amount, bottom_shift, is as much as 63.

Cast PAGE_SIZE to paddr_t so it has the right width.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Coverity ID: 1662707
Fixes: bac2000063 ('x86-64: reduce range spanned by 1:1 mapping and frame table indexes')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/common/pdx.c

@@ -274,7 +274,7 @@ bool __init pfn_pdx_compression_setup(paddr_t base)
 
     pfn_pdx_hole_shift  = hole_shift;
     pfn_pdx_bottom_mask = (1UL << bottom_shift) - 1;
-    ma_va_bottom_mask   = (PAGE_SIZE << bottom_shift) - 1;
+    ma_va_bottom_mask   = ((paddr_t)PAGE_SIZE << bottom_shift) - 1;
     pfn_hole_mask       = ((1UL << hole_shift) - 1) << bottom_shift;
     pfn_top_mask        = ~(pfn_pdx_bottom_mask | pfn_hole_mask);
     ma_top_mask         = pfn_top_mask << PAGE_SHIFT;