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 <[email protected]>
Cc: Greg Ungerer <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: David Howells <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
1 file changed