net/udp_offload: Handle static checker complaints

Fixed few issues around using __rcu prefix and rcu_assign_pointer, also
fixed a warning print to use ntohs(port) and not htons(port).

net/ipv4/udp_offload.c:112:9: error: incompatible types in comparison expression (different address spaces)
net/ipv4/udp_offload.c:113:9: error: incompatible types in comparison expression (different address spaces)
net/ipv4/udp_offload.c:176:19: error: incompatible types in comparison expression (different address spaces)

Signed-off-by: Shlomo Pongratz <shlomop@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>