mount-util: shorten the loop a bit (#7545)

mount-util: shorten the loop a bit (#7545)

The loop preparation and part of the loop contents are actually the
same, let's merge this.

Also, it's so much fun tweaking around in the name_to_handle_at() code,
let's do more of it with this patch!

(This also adds two NULL assignments, that aren't strictly necessary.
However, I figured its safer to place them in there, just in case the
for() condition is changed later. After all the freeing of the handle
and the invalidation of the cleanup-controller pointer to it are
otherwise really far away from each other...)