time/namespace: Validate nanosecond field in proc_timens_set_offset()

In the Linux kernel, the following vulnerability has been resolved: time/namespace: Validate nanosecond field in proc_timens_set_offset() The function validates tv_sec to be within [-KTIME_SEC_MAX, KTIME_SEC_MAX] but never validates that tv_nsec is within the valid range of [0, NSEC_PER_SEC-1] before using it in timespec64_add(). timespec64_add() expects both timespec64 structures to have normalized values with tv_nsec in the range [0, 999999999]. If off->val.tv_nsec contains invalid values (negative or >= NSEC_PER_SEC), it could lead to incorrect calculations or unexpected behavior. Add validation to ensure tv_nsec is within the valid range before performing the addition.