mlxsw: spectrum_router: Ignore routes using a deleted nexthop object

Routes are currently processed from a workqueue whereas nexthop objects
are processed in system call context. This can result in the driver not
finding a suitable nexthop group for a route and issuing a warning [1].

Fix this by ignoring such routes earlier in the process. The subsequent
deletion notification will be ignored as well.

[1]
 WARNING: CPU: 2 PID: 7754 at drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:4853 mlxsw_sp_router_fib_event_work+0x1112/0x1e00 [mlxsw_spectrum]
 [...]
 CPU: 2 PID: 7754 Comm: kworker/u8:0 Not tainted 5.11.0-rc6-cq-20210207-1 #16
 Hardware name: Mellanox Technologies Ltd. MSN2100/SA001390, BIOS 5.6.5 05/24/2018
 Workqueue: mlxsw_core_ordered mlxsw_sp_router_fib_event_work [mlxsw_spectrum]
 RIP: 0010:mlxsw_sp_router_fib_event_work+0x1112/0x1e00 [mlxsw_spectrum]

Fixes: cdd6cfc5 ("mlxsw: spectrum_router: Allow programming routes with nexthop objects")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reported-by: Alex Veber <alexve@nvidia.com>
Tested-by: Alex Veber <alexve@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>