netfilter: nf_tables: do not update stateful expressions if lookup is inverted

Initialize set lookup matching element to NULL. Otherwise, the
NFT_LOOKUP_F_INV flag reverses the matching logic and it leads to
deference an uninitialized pointer to the matching element. Make sure
element data area and stateful expression are accessed if there is a
matching set element.

This patch undoes 24791b9a ("netfilter: nft_set_bitmap: initialize set
element extension in lookups") which is not required anymore.

Fixes: 339706bc ("netfilter: nft_lookup: update element stateful expression")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>