mlxsw: spectrum_router: Introduce FIB event queue instead of separate works

Currently, every FIB event is queued-up as a separate work to be
processed. However, that allows to process only one FIB entry per work
callback.

In preparation of future XMDR register bulking of multiple FIB entries,
convert to FIB event queue. Implement this by a list_head, adding new
events to the end of the list in the FIB notify callback. That allows to
process multiple events from the list inside the work callback.

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