alloc-util: reintroduce malloc_usable_size() into greedy_realloc()

alloc-util: reintroduce malloc_usable_size() into greedy_realloc()

This is another attempt at d4b604baeadbb2498e4f2c3e260260eed210f5d6 and #12438

Instead of blindly using the extra allocated space, let's do so only
after telling libc about it, via a second realloc(). The second
realloc() should be quick, since it never has to copy memory around.