netfilter: nf_tables: Avoid duplicate call to nft_data_uninit() for same key

nft_del_setelem() currently calls nft_data_uninit() twice on the same
key. Once to release the key which is guaranteed to be NFT_DATA_VALUE
and a second time in the error path to which it falls through.

The second call has been harmless so far though because the type
passed is always NFT_DATA_VALUE which is currently a no-op.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>