netfilter: nft_set_rbtree: fix bogus EEXIST with NLM_F_CREATE with null interval

netfilter: nft_set_rbtree: fix bogus EEXIST with NLM_F_CREATE with null interval

Userspace adds a non-matching null element to the kernel for historical
reasons. This null element is added when the set is populated with
elements. Inclusion of this element is conditional, therefore,
userspace needs to dump the set content to check for its presence.

If the NLM_F_CREATE flag is turned on, this becomes an issue because
kernel bogusly reports EEXIST.

Add special case to ignore NLM_F_CREATE in this case, therefore,
re-adding the nul-element never fails.

Fixes: c016c7e45ddf ("netfilter: nf_tables: honor NLM_F_EXCL flag in set element insertion")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>