mpls: add missing unregister_netdevice_notifier to mpls_init

mpls: add missing unregister_netdevice_notifier to mpls_init

[ Upstream commit 99600f79b28c83c68bae199a3d8e95049a758308 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>