rtnetlink: Do not generate notifications for POST_TYPE_CHANGE event

Changing the master device for a link generates many messages; the one
generated for POST_TYPE_CHANGE is redundant:

[LINK]11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue master br1 state UNKNOWN group default
    link/ether 02:02:02:02:02:03 brd ff:ff:ff:ff:ff:ff
[LINK]11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue master br1 state UNKNOWN group default
    link/ether 02:02:02:02:02:03 brd ff:ff:ff:ff:ff:ff

Remove POST_TYPE_CHANGE from the list of notifiers that generate
notifications.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>