afr: remove unreachable halo threshold override branch

[ThalesBarretto]

Fixes: #4683

In __afr_handle_ping_event(), the inner check (up_children - 1) < priv->halo_min_replicas is algebraically unreachable when the outer guard up_children > priv->halo_min_replicas holds. The "Overriding halo threshold" log message is never emitted.

Algebraic proof

The outer guard requires up_children > halo_min_replicas. For non-negative integers, this implies up_children >= halo_min_replicas + 1, therefore up_children - 1 >= halo_min_replicas. The inner check (up_children - 1) < halo_min_replicas is the negation of this — always FALSE.

Signed/unsigned: up_children is int (computed by counting child_up[i] == 1, always non-negative). halo_min_replicas is uint32_t. The implicit int→unsigned promotion is safe because up_children is non-negative and both values are bounded by child_count. No overflow or sign-conversion hazard.

Provenance

Dead from birth. The outer guard has been > (strict greater-than) since the first halo prototype in 2014. See #4683 for the full git archaeology.

Date	Commit	Author	What happened
2014-04-29	bbceb48	Richard Wareing (FB)	First halo prototype. Outer guard > with inner check < min. Dead from birth.
2014-09-08	7a6ead5	Richard Wareing (FB)	Replaced the dead branch with find_worst_up_child(this) == idx — a reachable check.
2017-03-21	ca10139	Kevin Vigor (FB)	Squash-merged halo to upstream (Gerrit 16099 / 16177). Based on a version that predated Wareing's 7a6ead5 fix — the dead branch was re-introduced.
2020-02-04	4bb65d6	Pranith Kumar K (RH)	cluster/afr: Fixes for halo (bz#1800583). Added halo_child_up[] guards to the live branch but did not touch the dead branch.

Fix

Remove the dead branch, collapse the if/else. No behavior change — the dead branch only logged and never executed.

Build verified: make -C xlators/cluster/afr/src — zero errors, zero warnings.