Set the low water mark for hot pages in pcp to zero.

(akpm: for the life of me I cannot remember why we created pcp->low.  Neither
can Martin and the changelog is silent.  Maybe it was just a brainfart, but I
have this feeling that there was a reason.  If not, we should remove the
fields completely.  We'll see.)
Signed-off-by: Rohit Seth <rohit.seth@intel.com>
Cc: <linux-mm@kvack.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>