Fix isValid() condition for libsync tests

Upstream modifications in K3.12 to the sw_sync driver set O_CLOEXEC on
the fd's returned by the SW_SYNC_IOC_CREATE_FENCE ioctl, therefore we
check the return of fcntl() for a non-negative value instead of strictly 0.

Change-Id: If4c82750fcc0aa3a57f243fa2a94691e3150e4a4