lib: list_sort.c: Limit number of unused cmp callbacks

The helper merge_and_restore_back_links() makes sure to call the
caller's cmp function during the final ->prev pointer fixup, so that the
cmp function may call cond_resched().  However, if the cmp function does
not call cond_resched() at all, this is entirely redundant.  If it does,
doing at least two function calls for every two pointer assignments is a
bit excessive.  This patch limits the calls to once for every 256
iterations.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: Don Mullis <don.mullis@gmail.com>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>