sunrpc: Don't do a dst_confirm() on an input routes.

xs_udp_data_ready() is operating on received packets, and tries to
do a dst_confirm() on the dst attached to the SKB.

This isn't right, dst confirmation is for output routes, not input
rights.  It's for resetting the timers on the nexthop neighbour entry
for the route, indicating that we've got good evidence that we've
successfully reached it.

Signed-off-by: David S. Miller <davem@davemloft.net>