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

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 <[email protected]>
Cc: Artem Bityutskiy <[email protected]>
Cc: Don Mullis <[email protected]>
Cc: Dave Chinner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>