mpls: fix mpls_net_init memory leak

Fix a memory leak in the mpls netns init function in case of failure. If
register_net_sysctl fails then we need to free the ctl_table.

Fixes: 7720c01f ("mpls: Add a sysctl to control the size of the mpls label table")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>