netfilter: nft_ct: unnecessary to require dir when use ct l3proto/protocol

Currently, if the user want to match ct l3proto, we must specify the
direction, for example:
  # nft add rule filter input ct original l3proto ipv4
                                 ^^^^^^^^
Otherwise, error message will be reported:
  # nft add rule filter input ct l3proto ipv4
  nft add rule filter input ct l3proto ipv4
  <cmdline>:1:1-38: Error: Could not process rule: Invalid argument
  add rule filter input ct l3proto ipv4
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Actually, there's no need to require NFTA_CT_DIRECTION attr, because
ct l3proto and protocol are unrelated to direction.

And for compatibility, even if the user specify the NFTA_CT_DIRECTION
attr, do not report error, just skip it.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>