mpls: fix uninitialized in_label var warning in mpls_getroute

Fix the below warning generated by static checker:
    net/mpls/af_mpls.c:2111 mpls_getroute()
    error: uninitialized symbol 'in_label'."

Fixes: 397fc9e5 ("mpls: route get support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>