iscsi-target: use common inet_addr_is_any

Instead of open-coding it.

Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Cc: "Nicholas A. Bellinger" <nab@linux-iscsi.org>
Cc: target-devel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>