strncpy may left destination buffer not NULL terminated, switched using
snprintf to be sure destination buffer is NULL terminated.
Coverity issue: 1407495
Coverity issue: 1407498
Fixes: cc68ac4847bc ("net/af_packet: support MTU change")
Fixes: 218259590ea4 ("net/af_packet: support promiscuous")
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>