Commit 0af500a2 broke build on FreeBSD

Commit 0af500a2 started using __used for some quota struc, and there is already an existing macro on FreeBSD:

quota.c:27:23: error: expected identifier
        atomic_store(&quota->__used, 0);
                             ^
/usr/include/sys/cdefs.h:226:17: note: expanded from macro '__used'
#define __used          __attribute__((__used__))
                        ^

As a quick and dirty fix to fix the build, I s/__used/_used/, but it is probably not what you want to do.