bgpd: skip unsupported PREFIX_SID sub-type

bgpd: skip unsupported PREFIX_SID sub-type

This commit make bgpd to skip and ignore unsupported
sub-type of PREFIX_SID. (especially new defined sub-type)
Current bgpd can't parase unsupported sub-type of PREFIX_SID.
PREFIX_SID is drafted on draft-ietf-idr-bgp-prefix-sid-27.
There are already new sub-type drafted on
draft-dawra-idr-srv6-vpn-05. (Type5,6 is new defined.)
This commit fix the problem reported as #5277 on GitBub.

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>