Namecache lock+ref leaked on nc_vp==NULL error path (missing nlookup_done)

Summary

getsynthvnode(:82-84) returns NULL when nc_vp==NULL WITHOUT calling nlookup_done(&nd). Leaks held namecache lock+reference. nlookup_done only reached on fall-through(:90). Repeated calls wedge subsequent nlookup on that name. Boot/admin kernel context only.