udp: Fix UDP short packet false positive

The UDP header pointer assignment must happen after calling
pskb_may_pull().  As pskb_may_pull() can potentially alter the SKB
buffer.

This was exposted by running multicast traffic through the NIU driver,
as it won't prepull the protocol headers into the linear area on
receive.

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>