netfilter: nft_set_rbtree: Introduce and use nft_rbtree_interval_start()

Replace negations of nft_rbtree_interval_end() with a new helper,
nft_rbtree_interval_start(), wherever this helps to visualise the
problem at hand, that is, for all the occurrences except for the
comparison against given flags in __nft_rbtree_get().

This gets especially useful in the next patch.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>