mlxsw: spectrum_router: Introduce fib_entry priv for low-level ops

Prepare for the low-level ops that need to store some data alongside
the fib_entry and introduce a per-fib_entry priv for ll ops.
The priv is reference counted as in the follow-up patch it is going
to be saved in pack() function and used later on in commit() even in
case the related fib_entry gets freed in the middle.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>