netfilter: nft_reject: fix endianness in dump function

The dump function in nft_reject_ipv4 was not converting a u32
field to network order before sending it to userspace, this
needs to happen for consistency with other nf_tables and
nfnetlink subsystems.

Signed-off-by: Eric Leblond <eric@regit.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>