zebra: dummy kernel "install" multicast routes

zebra: dummy kernel "install" multicast routes

This is a followup to 9511633 ("zebra: MBGP routes should not be
installed in the kernel"), which was correct in disabling MRIB routes
being installed in the kernel, yet broke the MRIB since now routes were
never marked as active.

Hence, push down the check into the kernel install functions, so that
the routes are still marked active.  At the same time, the FPM calls get
a check each since otherwise we'd bump the FPM interface on MRIB
updates.

Fixes: 9511633 ("zebra: MBGP routes should not be installed in the kernel")
Cc: Everton Marques <everton.marques@gmail.com>
Cc: Balaji G <balajig81@gmail.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>