netfilter: nft_set_rbtree: allow loose matching of closing element in interval

Allow to find closest matching for the right side of an interval (end
flag set on) so we allow lookups in inner ranges, eg. 10-20 in 5-25.

Fixes: ba0e4d99

 ("netfilter: nf_tables: get set elements via netlink")
Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>