Capture a timer's locker for each instance of a CancellableTimer so that
reassigning a tcpip.CancellableTimer does not cause a data race.
Reassigning a tcpip.CancellableTimer updates its underlying locker. When
a timer fires, it does a read of the timer's locker variable to lock it.
This read of the locker was not synchronized so a race existed where one
goroutine may reassign the timer (updating the locker) and another
handles the timer firing (attempts to lock the timer's locker).
Test: tcpip_test.TestCancellableTimerReassignment
PiperOrigin-RevId: 307499822
782041509f