segtree: Check ranges when deleting elements

segtree: Check ranges when deleting elements

Make sure any intervals to delete actually exist, otherwise reject the
command. Without this, it is possible to mess up rbtree contents:

| # nft list ruleset
| table ip t {
| 	set s {
| 		type ipv4_addr
| 		flags interval
| 		auto-merge
| 		elements = { 192.168.1.0-192.168.1.254, 192.168.1.255 }
| 	}
| }
| # nft delete element t s '{ 192.168.1.0/24 }'
| # nft list ruleset
| table ip t {
| 	set s {
| 		type ipv4_addr
| 		flags interval
| 		auto-merge
| 		elements = { 192.168.1.255-255.255.255.255 }
| 	}
| }

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>