mlxsw: spectrum_router: Use gen_pool for RIF index allocation

Currently, each router interface (RIF) consumes one entry in the RIFs
table and there are no alignment constraints. This is going to change in
subsequent patches where some RIFs will consume two table entries and
their indexes will need to be aligned to the allocation size (even).

Prepare for this change by converting the RIF index allocation to use
gen_pool with the 'gen_pool_first_fit_order_align' algorithm.

No Kconfig changes necessary as mlxsw already selects
'GENERIC_ALLOCATOR'.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>