x86/mm: Align macro defines

Bring PAGE_{SHIFT,SIZE,MASK} to the same indentation level as the rest
of the header.
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1449480268-26583-1-git-send-email-bp@alien8.deSigned-off-by: NThomas Gleixner <tglx@linutronix.de>