br: fix potential panic when reprocessing a packet (#1662)
br: fix potential panic when reprocessing a packet (#1662)

The panic is caused by a race between a gorutine resetting the packet
because refCnt was 1, and another goroutine reprocessing the packet.

Save/restore the refCnt when resetting the metadata of a packet to be
repreprocessed fixes the issue.
klausman/scion,klausman/scion,Oncilla/scion,netsec-ethz/scion,klausman/scion,Oncilla/scion,Oncilla/scion,klausman/scion,netsec-ethz/scion,netsec-ethz/scion,klausman/scion,Oncilla/scion,netsec-ethz/scion