ipv4: fix reply_dst leakage on arp reply

There are cases when the created metadata reply is not used. Ensure the
allocated memory is freed also in such cases.

Fixes: 63d008a4 ("ipv4: send arp replies to the correct tunnel")
Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>