iprange: warn on reverse range (log)

Reverse ranges like B-A cause packets to be generally never matched,
as an address S does not match >=B && <=A (except for the border case
where S=A=B).

The kernel module itself does not check for reverse ranges, and it
seems nicer to check that in userspace anyway.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>