netfilter: ebtables: fix wrong name length while copying to user-space

user-space ebtables expects 32 bytes-long names, but xt_match names
use 29 bytes. We have to copy less 29 bytes and then, make sure we
fill the remaining bytes with zeroes.

Signed-off-by: Santosh Nayak <santoshprasadnayak@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>