fix bug in IPv6 payload checksum generation
fix bug in IPv6 payload checksum generation

When determining whether a checksum must be generated for the payload
look at IPV6.p not IPV6.nxt.  IPV6.p is the protocol number of the
payload, while IPV6.nxt is the protocol number of the next extension
header (or payload if there is no extension header).

Signed-off-by: Richard Hansen <b2eb39c2540dc27c75dc44a8dc9f96af73df0e80@rhansen.org>

hexcap/dpkt,smutt/dpkt,bpanneton/dpkt,lkash/test,jameslittle/dpkt,jack8daniels2/dpkt,yangbh/dpkt