net: ipv4: Fix packet leak with IPv4 autoconf

The IPv4 autoconfiguration feature relies on the fact, that autoconf
ARP packets are always prepared by the ARP module. After recent ARP
refactoring though that could no longer be the case due to packet
queueing mechanism. This could lead to net pkt leaks in the autoconf
module.

Fix this by skipping the pending packet queue for autoconf packets.
Since for autoconf ARP requests there's no really a pending packet
to queue, it can be safely avoided. This results in the ARP request
being always sent for the autoconf case, preventing the packet leak.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>