mm/highmem: make nr_free_highpages() return "unsigned long"

mm/highmem: make nr_free_highpages() return "unsigned long"

It looks rather weird that totalhigh_pages() returns an "unsigned long"
but nr_free_highpages() returns an "unsigned int".

Let's return an "unsigned long" from nr_free_highpages() to be consistent.

While at it, use a plain "0" instead of a "0UL" in the !CONFIG_HIGHMEM
totalhigh_pages() implementation, to make these look alike as well.

Link: https://lkml.kernel.org/r/20240607083711.62833-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>