intervals: do not report exact overlaps for new elements

intervals: do not report exact overlaps for new elements

Two new elements that represent an exact overlap should not trigger an error.

   add table t
   add set t s { type ipv4_addr; flags interval; }
   add element t s { 1.0.1.0/24 }
   ...
   add element t s { 1.0.1.0/24 }

result in a bogus error.

 # nft -f set.nft
 set.nft:1002:19-28: Error: conflicting intervals specified
 add element t s { 1.0.1.0/24 }
                   ^^^^^^^^^^

Fixes: 3da9643fb9ff ("intervals: add support to automerge with kernel elements")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>