mup: fix decode/serialize prefix in Interwork Segment Discovery Route

[higebu]

This PR fixes decoding and serializing the Prefix in Interwork Segment Dicsovery Route. The Prefix should be encoded as RFC 4760 5. NLRI Encoding: https://www.rfc-editor.org/rfc/rfc4760.html#section-5

b) Prefix:

  The Prefix field contains an address prefix followed by enough
  trailing bits to make the end of the field fall on an octet
  boundary.  Note that the value of trailing bits is irrelevant.

[fujita]

thanks!