Double-kfree/interior-pointer kfree in ng_fec_constructor error paths: heap corruption

Summary

ng_fec_constructor: ifp=&priv->arpcom.ac_if(:1085) ifp is EMBEDDED in priv NOT separately allocated. Both error paths kfree(ifp)(:1093/:1101) then kfree(priv)(:1094/:1102). kfree of interior pointer = UB for slab allocator -> panic or free list corruption. Subsequent kfree(priv) double-frees same allocation. Triggered by ENOMEM in ng_fec_get_unit or failure in ng_make_node_common under memory pressure. Fix: delete kfree(ifp) lines, only kfree(priv).