Fix data race in synRcvdState.
Fix data race in synRcvdState.

When checking the length of the acceptedChan we should hold the
endpoint mutex otherwise a syn received while the listening socket
is being closed can result in a data race where the cleanupLocked
routine sets acceptedChan to nil while a handshake goroutine
in progress could try and check it at the same time.

PiperOrigin-RevId: 251537697

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor