netfilter: nft_reject: unify reject init and dump into nft_reject

Bridge family is using the same static init and dump function as inet.

This patch removes duplicate code unifying these functions body into
nft_reject.c so they can be reused in the rest of families supporting
reject verdict.

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