mm/rmap: calculate page offset when needed

Call page_to_pgoff() to get the page offset once we are sure we actually
need it, and any very obvious initial function checks have passed.
Trivial micro-optimization, and potentially save some cycles.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>