slob: fix memory corruption

Previously, it would be possible for prev->next to point to
&free_slob_pages, and thus we would try to move a list onto itself, and
bad things would happen.

It seems a bit hairy to be doing list operations with the list marker as
an entry, rather than a head, but...

this resolves the following crash:

http://bugzilla.kernel.org/show_bug.cgi?id=9379

Signed-off-by: Nick Piggin <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Acked-by: Matt Mackall <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/slob.c

···
 		/* Improve fragment distribution and reduce our average
 		 * search time by starting our next search here. (see
 		 * Knuth vol 1, sec 2.5, pg 449) */
-		if (free_slob_pages.next != prev->next)
+		if (prev != free_slob_pages.prev &&
+				free_slob_pages.next != prev->next)
 			list_move_tail(&free_slob_pages, prev->next);
 		break;
 	}