MAINTAINERS: pass on hwpoison maintainership to Naoya Horiguchi

Horiguchi-san has done most of the work on hwpoison in the last years
and he also does most of the reviewing.  So I'm passing on the hwpoison
maintainership to him.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>