staticd: fix changing to source auto in bfd monitor

staticd: fix changing to source auto in bfd monitor

When monitoring a static route with BFD multi-hop, the source IP can be
either configured or retrieved from NextHop-Tracking (NHT). After
removing a configured source, the source is supposed to be retrieved
from NHT but it remains to the previous value. This is problematic if
the user desires to fix the configuration of a incorrect source IP.

For example, theses two commands results in the incorrect state:

> ip route 10.0.0.0/24 10.1.0.1 bfd multi-hop source 10.2.2.2
> ip route 10.0.0.0/24 10.1.0.1 bfd multi-hop

When removing the source, BFD is unable to find the source from NHT via
bfd_nht_update() were called.

Force zebra to resend the information to BFD by unregistering and
registering again NHT. The (...)/frr-nexthops/nexthop northbound
apply_finish function will trigger a call to static_install_nexthop()
that does a call to static_zebra_nht_register(nh, true);

Fixes: b7ca809d1c ("lib: BFD automatic source selection")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>