panic: lock (ng_worklist) sleep mutex does not match earlier (spin mutex) lock

[pluknet]

While debugging I noticed that sys/netgraph/ng_base.c#rev1.131
was MFCed to RELENG_6 inbeetwen 6.3 and 6.4 by mav as 1.102.2.15.

But this depends on sys/kern/subr_witness.c#rev1.227 which was
not MFCed, and that is triggering panic (in subj) if kernel is built
with WITNESS.

-- 
wbr,
pluknet