Stopped tx_thread_relinquish discarding the rebalance it requests

[fdesbiens]

Fixes #756

_tx_thread_relinquish walks the ready list at the relinquishing thread's priority for a thread to hand its core to. When the walk reaches one it cannot place, carrying preemption-threshold or excluded from that thread's mapped core, it sets the rebalance flag and breaks. That exit falls into the block concluding no other thread is ready, which is not what the walk found, and that block sets the finished flag. The rebalance at the end of the function is guarded on that flag, so it never runs.

Every rebalance requested from inside the walk is discarded, because that exit always reaches the block first. A ready thread behind the obstacle then never reaches a core while the others hold them and relinquish to each other. The tick keeps arriving, so it is a livelock rather than a deadlock.

The early return now applies only when no rebalance was requested, which lets control reach the rebalance path the other three request sites already use. No new code path.

threadx_smp_relinquish_rebalance_test ships with it: four threads of one priority relinquish in a loop holding the cores, a thread excluded from every core sits behind them, and a runnable thread behind that must run within 100 ticks.

It fails 3 of 3 before this change and passes 5 of 5 after. Instrumenting a passing run of threadx_thread_relinquish_test counted 12 rebalance requests from the walk against 4 performed, and the 4 are reachable only from the request sites outside it.