from the interface index, then decrease refcount, not vice versa.
Otherwise there is a race (reproducible) when if_free_internal()
contests on IFNET_WLOCK(), and we got a zero-refed ifnet in the
index for a long time. It may be picked by some other thread,
that runs ifnet_byindex_ref(), who takes the ifnet from index,
and bumps refcount. When reader drops the lock, if_free_internal()
proceeds with free. Then reader tries to free it a second time.
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