bgpd: announce locators instead of transposed SIDs

bgpd: announce locators instead of transposed SIDs

This patch changes the format of the Prefix-SID advertised by
bgpd. In current implementation, transposed SIDs were
advertised, which caused two problems:

1. bgpd that receives SRv6 L3VPN routes whose SID is
   transposed couldn't put bgp_attr_srv6_l3vpn whose those
   routes together. This leads extra memory consumption.

2. Some implementation will reject a route with transposed SID.
   This will affect interoperability.

For those reasons, in this patch, instead of advertising
transposed SID, we change it to advertise the locator of SID.

Signed-off-by: Ryoga Saito <ryoga.saito@linecorp.com>