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

This reverts commit 916f9685fa.

xfrm_monitor_linux.go

@@ -2,7 +2,6 @@ package netlink
 
 import (
 	"fmt"
-	"syscall"
 
 	"github.com/vishvananda/netlink/nl"
 	"github.com/vishvananda/netns"
@@ -57,9 +56,6 @@ func XfrmMonitor(ch chan<- XfrmMsg, done <-chan struct{}, errorChan chan<- error
 		for {
 			msgs, from, err := s.Receive()
 			if err != nil {
-				if err == syscall.EAGAIN {
-					continue
-				}
 				errorChan <- err
 				return
 			}