net/kni: fix config initialization

net/kni: fix config initialization

When kni driver calls eth_kni_start to start device, some fields such as
min_mtu and max_mtu of rte_kni_conf are not initialized. It will cause
kni_ioctl_create create a kni netdevice with a random min_mtu and
max_mtu value. This is unexpected and sometimes we could not change the
kni device mtu with ip link command.

Fixes: ff1e35fb5f83 ("kni: calculate MTU from mbuf size")
Cc: stable@dpdk.org
Signed-off-by: Harold Huang <baymaxhuang@gmail.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>