device: serialize access to IpcSetOperation

Interleaves IpcSetOperations would spell trouble.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/device.go

@@ -23,6 +23,7 @@ type Device struct {
 	isUp     AtomicBool // device is (going) up
 	isClosed AtomicBool // device is closed? (acting as guard)
 	log      *Logger
+	ipcSetMu sync.Mutex // serializes UAPI set operations
 
 	// synchronized resources (locks acquired in order)