mpls: small cleanup in inet/inet6_fib_lookup_dev()

We recently changed this code from returning NULL to returning ERR_PTR.
There are some left over NULL assignments which we can remove.  We can
preserve the error code from ip_route_output() instead of always
returning -ENODEV.  Also these functions use a mix of gotos and direct
returns.  There is no cleanup necessary so I changed the gotos to
direct returns.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>