malloc_usable_size does not down-adjust to reflect the size usable by the caller, i.e. minus the insert space

[stephenrkell]

The new libmallochooks contains a hook path for malloc_usable_size, and correspondingly we have dropped our ad-hoc preload (in fdc7ef4). However, we need this hook path to tweak the size conversely to how it is incremented during allocation. Currently I think the incremented size just gets passed through. This was also the case with the ad-hoc preload I think (not sure; it did delegate to the allocator's get_size function).

This is an easy-ish fix (mostly in generic_malloc_index.h I think)... just leaving it here while I remember.