malloc: Fix missing accounting of top chunk in malloc_info [BZ #24026]

Fixes `<total type="rest" size="..."> incorrectly showing as 0 most
of the time.

The rest value being wrong is significant because to compute the
actual amount of memory handed out via malloc, the user must subtract
it from <system type="current" size="...">. That result being wrong
makes investigating memory fragmentation issues like
<https://bugzilla.redhat.com/show_bug.cgi?id=843478> close to
impossible.

(cherry picked from commit b6d2c447)