x86: pgd_{c,d}tor() cleanup

Giving pgd_ctor() a properly typed parameter allows eliminating a local
variable. Adjust pgd_dtor() to match.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: "Jeremy Fitzhardinge" <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>