nat: test all intf addrs in is_interface_addr()

nat: test all intf addrs in is_interface_addr()

Type: fix

Currently, is_interface_addr() tests if a given IPv4 address belongs to
an interface by a given sw_if_index. However, there are several issues:
 * only the first found address on the interface is actually tested,
 * sw_if_index is always cached even if the interface hasn't been
   assigned any addresses yet.

With this change, is_interface_addr() tests all IPv4 addresses on an
interface by a given sw_if_index and caches sw_if_index only if there
are addresses present.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>
Change-Id: If1acc4a534647a5f0ce8e9b565b867c92a016dc3