`for_each_mem_cluster(x, y, z)' will expand to
`for ((x) = (y)->x ...' but correct is `for ((x) = (y)->cluster ...'

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>