The idea of nm_free_secret() is to clear the secrets from memory. That
surely is some layer of extra snake oil, because we tend to pass secrets
via D-Bus, where the memory gets passed down to (D-Bus) libraries which
have no idea to keep it private. Still...
But turns out, malloc_usable_size() might not actually be usable for
this. Read the discussion at [1].
Stop using malloc_usable_size(), which seems unfortunate.
There is probably no secret relevant data after the NUL byte anyway,
because we tend to create such strings once, and don't rewrite/truncate
them afterwards (which would leave secrets behind as garbage).
Note that systemd's erase_and_free() still uses malloc_usable_size()
([2]) but the macro foo to get that right is terrifying ([3]).
[1] https://github.com/systemd/systemd/issues/22801#issuecomment-1343041481
[2] https://github.com/systemd/systemd/blob/11c0f0659ecd82572c2dc83f3b34493a36dcd954/src/basic/memory-util.h#L101
[3] https://github.com/systemd/systemd/commit/7929e180aa47a2692ad4f053afac2857d7198758
Fixes: d63cd26e6042 ('shared: improve nm_free_secret() to clear entire memory buffer')
(cherry picked from commit 8b66865a88af7bf835147d66c309dae008507c36)