If allocating a TX kring's tx_pool fails, the error path destroys the
tx_event_lock of every TX kring, including those whose lock was never
initialized, which panics on INVARIANTS kernels.
Only destroy the tx_event_lock for krings that have initialized it.
Reported by: llm-postcr
Fixes: 37e3a6d34958 ("Import the current version of netmap, aligned with the one on github.")