netfilter: nf_tables: missing error reporting for not selected expressions

Sometimes users forget to turn on nftables extensions from Kconfig that
they need. In such case, the error reporting from userspace is
misleading:

 $ sudo nft add rule x y counter
 Error: Could not process rule: No such file or directory
 add rule x y counter
 ^^^^^^^^^^^^^^^^^^^^

Add missing NL_SET_BAD_ATTR() to provide a hint:

 $ nft add rule x y counter
 Error: Could not process rule: No such file or directory
 add rule x y counter
              ^^^^^^^

Fixes: 83d9dcba

 ("netfilter: nf_tables: extended netlink error reporting for expressions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>