kshareddatacache: Use proper return value check for timedlock.

I discovered that the return value was being checked as if
pthread_mutex_timedlock used errno for return values while doing a code
review for bug 305023. In reality the only non-error return is 0,
anything else is an error code which is returned directly.

I'm making the fix now so it can join KDE Platform 4.9.1.