x64/page.h: convert some macros to inlines

Convert clear_page/copy_page macros to inline functions for type-checking.
Andrew wants to extirpate these ugly macros. (Ingo too. Thomas as well.
Please send us more "kill ugly macros" patches! :-)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>