pppolac: pull udp header before sock enqueue

pppolac driver incorrectly enqueues the packet into the sock queue
without pulling UDP headers. The application will receive data along
with UDP header when L2TP control packets are received.

The issue was introduced after moving UDP header removal functionality
from process rcvmesg context to BH context.

Instead of pppolac driver directly queuing L2TP control packets into
socket queue, return packet to udp_queue_rcv_skb, which will deliver the
packet to the application after pulling the UDP header.

Fixes: e6afc8ac ("udp: remove headers from UDP packets before queueing")

Change-Id: Icfa0fd8da43ea9c14fa7c718746a6529651ac202
Acked-by: Sharath Chandra Vurukala <sharathv@qti.qualcomm.com>
Signed-off-by: Tejaswi Tanikella <tejaswit@codeaurora.org>