fi_av_insert() is invoked with a context containing each endpoint
USNIC_NUM_CHANNELS times. If the address on that endpoint fails to
resolve / is unreachable / has some error, we'll therefore get
USNIC_NUM_CHANNELS error completions with that same endpoint. We
therefore only want to warn about the unreachability of (and
OBJ_RELEASE) that endpoint the *first* time.
Fixes CSCut46822.
6da77ee940