quorum: don't allow quorum_trackstart to be called twice

If quorum_trackstart() or votequorum_trackstart() are called twice with
CS_TRACK_CHANGES then the client gets added twice to the notifications
list effectively corrupting it. Users have reported segfaults in
corosync when they did this (by mistake!).

As there's already a tracking_enabled flag in the private-data, we check
that before adding to the list again and return an error if
the process is already registered.

Signed-off-by: Christine Caulfield <ccaulfie@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>