Fix Segment Routing Pad1 TLV definition

[yiyuandao]

In RFC 8754， 2.1.1.1, Pad1 is a single byte.

   0 1 2 3 4 5 6 7
  +-+-+-+-+-+-+-+-+
  |     Type      |
  +-+-+-+-+-+-+-+-+

but in the current implementation, Pad1 are three bytes, b'\x00\x01\x00'

raw(IPv6ExtHdrSegmentRoutingTLVPad1())

This PR fixed it.

Hope it can help.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.70%. Comparing base (a8583a5) to head (53d006b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4611      +/-   ##
==========================================
- Coverage   81.38%   78.70%   -2.69%     
==========================================
  Files         359      334      -25     
  Lines       86029    80830    -5199     
==========================================
- Hits        70017    63614    -6403     
- Misses      16012    17216    +1204     

Files with missing lines	Coverage Δ
scapy/layers/inet6.py	87.81% <100.00%> (-0.73%)	⬇️

... and 285 files with indirect coverage changes