include/asm-generic/page.h: calculate virt_to_page and page_to_virt via predefined macro

On NOMMU architectures, if physical memory doesn't start from 0,
ARCH_PFN_OFFSET is defined to generate page index in mem_map array.
Because virtual address is equal to physical address, PAGE_OFFSET is
always 0.  virt_to_page and page_to_virt should not index page by
PAGE_OFFSET directly.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Cc: Greg Ungerer <gerg@snapgear.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>