mm/readahead.c, mm/vmscan.c: use lru_to_page instead of list_to_page

list_to_page() in readahead.c is the same as lru_to_page() in vmscan.c.
So I move lru_to_page to internal.h and drop list_to_page().

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>