sh: use PFN_DOWN macro

Replace ((x) >> PAGE_SHIFT) with the predefined PFN_DOWN macro.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>