x86/paravirt: fix missing callee-save call on pud_val

Impact: Fix build when CONFIG_PARAVIRT_DEBUG is enabled

Fix missed convertion to using callee-saved calls for pud_val, which
causes a compile error when CONFIG_PARAVIRT_DEBUG is enabled.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>