ng_fec_free_unit increments usage counter instead of decrementing + global bitmap mutated without locks

Summary

ng_fec_get_unit increments ng_units_in_use(:290). ng_fec_free_unit ALSO increments it(:315 ng_units_in_use++ should be --). Counter never decreases. if(ng_units_in_use==0)(:316) cleanup branch dead code. Signed overflow monotonic int UB. Separately ng_fec_units/len/in_use global NO lock (code notes XXX make SMP safe :316). Concurrent constructor/rmnode race on bitmap+realloc. Unit number collision, memory leak, allocator UB. Fix: change :315 to ng_units_in_use--, add lock.