mpls: add missing unregister_netdevice_notifier to mpls_init

If mpls_init() fails after registering mpls_dev_notifier, it never
gets removed. Add the missing unregister_netdevice_notifier() call to
the error handling path.

Fixes: 5be2062e3080 ("mpls: Handle error of rtnl_register_module().")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/7c55363c4f743d19e2306204a134407c90a69bbb.1773228081.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>