CFT: vr(4)

It seems that there had been several stability issues in vr(4).
Here is mimimal patch that make vr(4) work reliably under heavy
network loads. The patch does the following:
 - Always check writability of mbuf before padding and make a
   writable copy of the mbuf if mbuf is marked as read-only.
 - Before padding is done check remaining bytes such that it can
   safely extend buffer size of the mbuf.
 - Before padding always check the return value of m_defrag(9).
 - Zero out pad space to avoid leaking data.

If you have vr(4) hardware please give it spin and let me know
the result.

-- 
Regards,
Pyun YongHyeon