pkg/ns: fixed the check for network namespace path.

The expectation on older kernels (< 3.19) was to have the network
namespace always be a directory. This is not true if the network
namespace is bind mounted to a file, and will make the plugin fail
erroneously in such cases. The fix is to remove this assumption
completely and just do a basic check on the file system types being
returned.

Fixes #288

pkg/ns/ns.go

@@ -20,7 +20,6 @@ import (
 	"os"
 	"path"
 	"runtime"
-	"strings"
 	"sync"
 	"syscall"
 
@@ -101,19 +100,7 @@ func IsNSorErr(nspath string) error {
 	}
 
 	switch stat.Type {
-	case PROCFS_MAGIC:
-		// Kernel < 3.19
-
-		validPathContent := "ns/"
-		validName := strings.Contains(nspath, validPathContent)
-		if !validName {
-			return NSPathNotNSErr{msg: fmt.Sprintf("path %q doesn't contain %q", nspath, validPathContent)}
-		}
-
-		return nil
-	case NSFS_MAGIC:
-		// Kernel >= 3.19
-
+	case PROCFS_MAGIC, NSFS_MAGIC:
 		return nil
 	default:
 		return NSPathNotNSErr{msg: fmt.Sprintf("unknown FS magic on %q: %x", nspath, stat.Type)}