Our list_sort() utility has always supported a context argument that
is passed through to the comparison routine. Now there's a use case
for the similar thing for sort().

This implements sort_r by simply extending the existing sort function
in the obvious way. To avoid code duplication, we want to implement
sort() in terms of sort_r(). The naive way to do that is

static int cmp_wrapper(const void *a, const void *b, const void *ctx)
{
int (*real_cmp)(const void*, const void*) = ctx;
return real_cmp(a, b);
}

sort(..., cmp) { sort_r(..., cmp_wrapper, cmp) }

but this would do two indirect calls for each comparison. Instead, do
as is done for the default swap functions - that only adds a cost of a
single easily predicted branch to each comparison call.

Aside from introducing support for the context argument, this also
serves as preparation for patches that will eliminate the indirect
comparison calls in common cases.

Requested-by: Boris Brezillon

Signed-off-by: Rasmus Villemoes
Signed-off-by: Boris Brezillon
Acked-by: Andrew Morton
Tested-by: Philipp Zabel
Signed-off-by: Hans Verkuil
Signed-off-by: Mauro Carvalho Chehab