netfilter: nf_dup: fix sparse warnings

>> net/ipv4/netfilter/nft_dup_ipv4.c:29:37: sparse: incorrect type in initializer (different base types)
   net/ipv4/netfilter/nft_dup_ipv4.c:29:37:    expected restricted __be32 [user type] s_addr
   net/ipv4/netfilter/nft_dup_ipv4.c:29:37:    got unsigned int [unsigned] <noident>

>> net/ipv6/netfilter/nf_dup_ipv6.c:48:23: sparse: incorrect type in assignment (different base types)
   net/ipv6/netfilter/nf_dup_ipv6.c:48:23:    expected restricted __be32 [addressable] [assigned] [usertype] flowlabel
   net/ipv6/netfilter/nf_dup_ipv6.c:48:23:    got int

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>