net: stmmac: selftests: fix potential memleak in stmmac_test_arpoffload()

The skb allocated by stmmac_test_get_arp_skb() hasn't been released in
some error handling case, which will lead to a memory leak. Fix this up
by adding kfree_skb() to release skb.

Compile tested only.

Fixes: 5e3fb0a6

 ("net: stmmac: selftests: Implement the ARP Offload test")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>