netfilter: nft_dup: do not use sreg_dev if the user doesn't specify it

The NFTA_DUP_SREG_DEV attribute is not a must option, so we should use it
in routing lookup only when the user specify it.

Fixes: d877f071 ("netfilter: nf_tables: add nft_dup expression")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>