Adding support for RFC8227 withdraw label

According to RFC8227, many vendors don't properly implement RFC3107
related to withdraw label. Indeed, it happens that a speaker sends
a label of 0x000000 in place of 0x800000 to signal a route withdrawal.
As a result, the parser fails to parse the message since it doesn't stop
at the withdraw label and tries to parse more label than there is.

This commit fixes this by considering both 0x800000 and 0x000000 as withdraw
labels.

ryu/lib/packet/bgp.py

@@ -782,7 +782,16 @@ class _LabelledAddrPrefix(_AddrPrefix):
     # Routes field should be set to 0x800000.  (Of course, terminating the
     # BGP session also withdraws all the previously advertised routes.)
     #
-    _WITHDRAW_LABEL = 0x800000
+    # RFC8227
+    # 2.4 How to Explicitly Withdraw the Binding of a Label to a Prefix
+    # [RFC3107] also made it possible to withdraw a binding without specifying
+    # the label explicitly, by setting the Compatibility field to 0x800000.
+    # However, some implementations set it to 0x000000. In order to ensure
+    # backwards compatibility, it is RECOMMENDED by this document that the
+    # Compatibility field be set to 0x800000, but it is REQUIRED that it be
+    # ignored upon reception.
+    #
+    _WITHDRAW_LABELS = [0x800000, 0x000000]
 
     def __init__(self, length, addr, labels=None, **kwargs):
         labels = labels if labels else []
@@ -823,7 +832,7 @@ def _to_bin(cls, addr):
         labels = addr[0]
         rest = addr[1:]
         labels = [x << 4 for x in labels]
-        if labels and labels[-1] != cls._WITHDRAW_LABEL:
+        if labels and labels[-1] not in cls._WITHDRAW_LABELS:
             labels[-1] |= 1  # bottom of stack
         bin_labels = list(cls._label_to_bin(l) for l in labels)
         return bytes(reduce(lambda x, y: x + y, bin_labels,
@@ -837,7 +846,7 @@ def _has_no_label(cls, bin_):
             while True:
                 (label, bin_) = cls._label_from_bin(bin_)
                 labels.append(label)
-                if label & 1 or label == cls._WITHDRAW_LABEL:
+                if label & 1 or label in cls._WITHDRAW_LABELS:
                     break
             assert length > struct.calcsize(cls._LABEL_PACK_STR) * len(labels)
         except struct.error:
@@ -857,7 +866,7 @@ def _from_bin(cls, addr):
         while True:
             (label, rest) = cls._label_from_bin(rest)
             labels.append(label >> 4)
-            if label & 1 or label == cls._WITHDRAW_LABEL:
+            if label & 1 or label in cls._WITHDRAW_LABELS:
                 break
         return (labels,) + cls._prefix_from_bin(rest)