ipsec_copypkt: use sys/kern/uipc_mbuf.c:m_dup() for netbsd
ipsec_copypkt: use sys/kern/uipc_mbuf.c:m_dup() for netbsd

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame