net/mlx5e: Fix loopback self test when GRO is off

When GRO is off, the transport header pointer in sk_buff is
initialized to network's header.

To find the udp header, instead of using udp_hdr() which assumes
skb_network_header was set, manually calculate the udp header offset.

Fixes: 0952da79

 ("net/mlx5e: Add support for loopback selftest")
Signed-off-by: Inbar Karmy <inbark@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>