mlxsw: spectrum_router: Have FIB entry op context allocated for the instance

Get the max size needed for FIB entry op context and allocate it once
for the instance. Use it repeatedly from the scheduled work.
By this, allow to extend the context to hold more data than it is wise
to do when it was on the stack. Make sure to signalize that the context
needs to be initialized in case families of subsequent FIB entries differ.

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