mpls: Send route delete notifications when router module is unloaded

mpls: Send route delete notifications when router module is unloaded

When the mpls_router module is unloaded, mpls routes are deleted but
notifications are not sent to userspace leaving userspace caches
out of sync. Add the call to mpls_notify_route in mpls_net_exit as
routes are freed.

Fixes: 0189197f44160 ("mpls: Basic routing support")
Signed-off-by: David Ahern <[email protected]>
Signed-off-by: David S. Miller <[email protected]>