net: arp: Fix failed to add the arp table in stress test

During stress test with WiFi connect, disconnect, ping and throughput
traffic, ARP table updating failed issue may occur.
In arp_prepare(), if packet allocate failed, should add the arp
entry back to arp_free_entries, to avoid this entry is leak forever.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>