[inet6] make ICMPv6NDOptUnknown a bit more fuzzable (#4305)

Without this patch the type of Neighbor Discovery options generated by
fuzz(ICMPv6NDOptUnknown()) is always 0. With this patch applied option
types are random.

It's a follow-up to #4233

scapy/layers/inet6.py

@@ -1789,7 +1789,7 @@ def m2i(self, pkt, x):
 
 class ICMPv6NDOptUnknown(_ICMPv6NDGuessPayload, Packet):
     name = "ICMPv6 Neighbor Discovery Option - Scapy Unimplemented"
-    fields_desc = [ByteField("type", None),
+    fields_desc = [ByteField("type", 0),
                    FieldLenField("len", None, length_of="data", fmt="B",
                                  adjust=lambda pkt, x: (2 + x) // 8),
                    ICMPv6NDOptDataField("data", "", strip_zeros=False,

test/scapy/layers/inet6.uts

@@ -837,6 +837,8 @@ assert raw(p) == b
 p = ICMPv6NDOptSrcLLAddr(b)[ICMPv6NDOptUnknown]
 assert p.type == 0 and p.len == 2 and p.data == b'somestring\x00\x00\x00\x00'
 
+= ICMPv6NDOptUnknown - fuzz
+assert isinstance(fuzz(ICMPv6NDOptUnknown()).type, RandByte)
 
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