bgpd: fix Prefix-SID parsing failure case

Prefix-SID path attribute Label-index TLV (type-1) is
used by SR-MPLS. And Label-index TLV MUST ignored
if that path attribute is append on non-Labeled-unicast
UPDATE message described on [ref1].
There is a problem case exist arround this implementation.
This commit fix that.

Before this commit,
unfortunally, setting Label-Index value is skipped at somecases.
because, Label-Index TLV implementation check the AFI/SAFI pair.
by mp_update variable that is set by bgp_mp_reach_parse function.
if MP_REACH_NLRI is present after PREFIX_SID, bgp_attr_psid_sub
function can't understand AFI/SAFI pair. and the order of each
path attributes is never no-deterministic thing for receiver.[ref2]

In this commit,
I re-located checking code of AFI/SAFI pair after path-attr loop.

[ref1](https://tools.ietf.org/html/draft-ietf-idr-bgp-prefix-sid-27#section-3.2)
> The Originator SRGB TLV may only appear in a BGP Prefix-SID attribute
> attached to IPv4/IPv6 Labeled Unicast prefixes ([RFC8277]).  It MUST
> be ignored when received for other BGP AFI/SAFI combinations.

[ref2](https://tools.ietf.org/html/rfc4271#section-5)
> The sender of an UPDATE message SHOULD order path attributes within
> the UPDATE message in ascending order of attribute type.  The
> receiver of an UPDATE message MUST be prepared to handle path
> attributes within UPDATE messages that are out of order.

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

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