NDP packets are sent periodically from NDP timers. These timers do not
hold the NIC lock when sending packets as the packet write operation
may take some time. While the lock is not held, the NIC may be removed
by some other goroutine. This change handles that scenario gracefully.
Test: stack_test.TestRemoveNICWhileHandlingRSTimer
PiperOrigin-RevId: 315524143