vrf: T5428: stop DHCP processes on VRF removal

This is a workaround for the priority inversion from T5492 ("CLI node priority
is not inversed on node deletion"). As this is a corner case bug that's only
triggered if an interface is removed from a VRF and also the VRF is removed in
one commit, priorities are not honored.

Thus we implement this workaround which stop the DHCP(v6) client processes on
the VRF associated interfaces to get out the DHCP RELEASE message before
interfaces are shut down.

(cherry picked from commit 005151f77be5cf999689cfd03620bbc39df59018)