netfilter: nf_tables: add nft_setelem_parse_key()

Add helper function to parse the set element key netlink attribute.

v4: No changes
v3: New patch

[sbrivio: refactor error paths and labels; use NFT_DATA_VALUE_MAXLEN
  instead of sizeof(*key) in helper, value can be longer than that;
  rebase]
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>