add WITNESS support to barriers modelled on the timeout stuff visa did.

if a taskq takes a lock, and something holding that lock calls
taskq_barrier, there's a potential deadlock. detect this as a lock
order problem when witness is enable. task_del conditionally followed
by taskq_barrier is a common pattern, so add a taskq_del_barrier
wrapper for it that unconditionally checks for the deadlock, like
timeout_del_barrier.

ok visa@

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