alloc-util: introduce MALLOC_SIZEOF_SAFE() helper

It's a wrapper around malloc_usable_size() that is supposed to be
compatible with _FORTIFY_SOURCES=1, by taking the
__builtin_object_size() data into account, the same way as the
_FORTIFY_SOURCES=1 logic does.

Fixes: #19203
(cherry picked from commit 6df28e1f)

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1975564