ctdb-daemon: Factor out new function release_ip_post()

ctdb-daemon: Factor out new function release_ip_post()

This contains the cleanup that needs to be done after an IP address is
released from an interface.

state->vnn is set to the return value from release_ip_post(), which is
either the original VNN, or NULL if it was deleted.  This allows
correct handling of the in-flight flag in the destructor for state.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12158

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>