netfilter: nft_ct: fix unconditional dump of 'dir' attr

netfilter: nft_ct: fix unconditional dump of 'dir' attr

We want to make sure that the information that we get from the kernel can
be reinjected without troubles. The kernel shouldn't return an attribute
that is not required, or even prohibited.

Dumping unconditionally NFTA_CT_DIRECTION could lead an application in
userspace to interpret that the attribute was originally set, while it
was not.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>