netfilter: nf_tables: do not set up extensions for end interval

Elements with an end interval flag set on do not store extensions. The
global set definition is currently setting on the timeout and stateful
expression for end interval elements.

This leads to skipping end interval elements from the set->ops->walk()
path as the expired check bogusly reports true.

Moreover, do not set up stateful expressions for elements with end
interval flag set on since this is never used.

Fixes: 65038428 ("netfilter: nf_tables: allow to specify stateful expression in set definition")
Fixes: 8d8540c4 ("netfilter: nft_set_rbtree: add timeout support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>