x86, pgtable.h: macro-ify *_page() methods

The p?d_page() methods still rely on highlevel types and methods:

In file included from arch/x86/kernel/early_printk.c:18:
/home/mingo/tip/arch/x86/include/asm/pgtable.h: In function ‘pmd_page’:
/home/mingo/tip/arch/x86/include/asm/pgtable.h:516: error: implicit declaration of function ‘__pfn_to_section’
/home/mingo/tip/arch/x86/include/asm/pgtable.h:516: error: initialization makes pointer from integer without a cast
/home/mingo/tip/arch/x86/include/asm/pgtable.h:516: error: implicit declaration of function ‘__section_mem_map_addr’
/home/mingo/tip/arch/x86/include/asm/pgtable.h:516: error: return makes pointer from integer without a cast

So convert them to macros and document the type dependency.

Signed-off-by: Ingo Molnar <mingo@elte.hu>