netfilter: nf_reject: add reject skbuff creation helpers

Adds reject skbuff creation helper functions to ipv4/6 nf_reject
infrastructure. Use these functions for reject verdict in bridge
family.

Can be reused by all different families that support reject and
will not inject the reject packet through ip local out.

Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>