alloc-util: don't use malloc_usable_size() to determine allocated size

This reverts commit d4b604ba.

When realloc() is called, the extra memory between the originally
requested size and the end of malloc_usable_size() isn't copied. (at
least with the version of glibc that currently ships on Arch Linux)
As a result, some elements get lost and use uninitialized memory, most
commonly 0, and can lead to crashes.

fixes #12384