intervals: use expression location when translating to intervals

intervals: use expression location when translating to intervals

Otherwise, internal location reports:

 # nft -f ruleset.nft
 internal:0:0-0: Error: Could not process rule: File exists

after this patch:

 # nft -f ruleset.nft
 ruleset.nft:402:1-16: Error: Could not process rule: File exists
 1.2.3.0/30,
 ^^^^^^^^^^^

Fixes: 81e36530fcac ("src: replace interval segment tree overlap and automerge")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>