malloc: Use mtag_enabled instead of USE_MTAG

Use the runtime check where possible: it should not cause slow down in
the !USE_MTAG case since then mtag_enabled is constant false, but it
allows compiling the tagging logic so it's less likely to break or
diverge when developers only test the !USE_MTAG case.

Reviewed-by: DJ Delorie <dj@redhat.com>