Revert "Change behavior of Subscribe to non-blocking. Fix test."

This reverts commit 916f9685fa.

link_linux.go

@@ -2434,9 +2434,6 @@ func linkSubscribeAt(newNs, curNs netns.NsHandle, ch chan<- LinkUpdate, done <-c
 		for {
 			msgs, from, err := s.Receive()
 			if err != nil {
-				if err == syscall.EAGAIN {
-					continue
-				}
 				if cberr != nil {
 					cberr(fmt.Errorf("Receive failed: %v",
 						err))