mm/hwpoison-inject: fix refcounting in no-injection case

Hwpoison injection via debugfs:hwpoison/corrupt-pfn takes a refcount of
the target page.  But current code doesn't release it if the target page
is not supposed to be injected, which results in memory leak.  This patch
simply adds the refcount releasing code.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Dean Nelson <dnelson@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>