for_each_online_pgdat(): kerneldoc fix

for_each_pgdat() was renamed to for_each_online_pgdat() and kerneldoc
comments should be updated accordingly.

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>