vrrpd: fix broken reads when reinitializing

vrrpd: fix broken reads when reinitializing

When a VRRP router was shut down - either due to an administrative
event, or its interface getting deleted, or some other reason - it was
forgetting to cancel its read task. When it was started again, the read
task was still around, and so it wasn't getting scheduled again with the
new socket fd's. This caused our socket to queue ingress packets but
never read them, resulting in the restarted router always electing
itself to Master (since it wasn't listening to any other advertisements,
even though the kernel was delivering them).

The t_write cancellation call doesn't matter here, but I'm putting it in
there because it doesn't hurt and this way I won't forget about it if it
becomes necessary in the future.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>