routing: Enable hash_outbound during nhgrp allocation

[pouria]

Summary

Multipath routes can be added via both RTM_F_CREATE and RTM_F_APPEND.
Therefore, it's possible to have mpath routes without calling
add_route_flags_mpath.

Instead of checking V_fib_hash_outbound for every route append,
check it during nhgrp_ctl initialization, which is only called for
the first multipath request per rib_head.

PR: 293136

Test Plan

1. Add multipath routes per vnet.
2. Verify sysctl net.route.hash_outbound.

[zarychtam_plan-b.pwste.edu.pl]

The approach proposed here seems preferable to moving the check into rib_add_route_px(), which I previously suggested as a solution for PR 293136 in Bugzilla.

While the patch proposed for PR 293136 works correctly, it introduces a small amount of overhead. Although minor, this overhead is now visible in the routing update processing path used by BIRD. In a recent flame graph captured on our ASBR running FreeBSD 15.1 and BIRD 3.3.0, it shows up as a slight regression.

Given that, I am inclined to support this simpler approach, as it appears to avoid the additional overhead and keeps route processing on the critical path as efficient as possible.

As I am only an observer on this review and not one of its reviewers, I will leave the final judgment to those more familiar with this part of the codebase.

[glebius]

Not an expert here, but after brief looks good.