mlxsw: spectrum_router: Remove inappropriate usage of WARN_ON()

commit d9d54202 upstream.

We should not trigger a warning when a memory allocation fails. Remove
the WARN_ON().

The warning is constantly triggered by syzkaller when it is injecting
faults:

[ 2230.758664] FAULT_INJECTION: forcing a failure.
[ 2230.758664] name failslab, interval 1, probability 0, space 0, times 0
[ 2230.762329] CPU: 3 PID: 1407 Comm: syz-executor.0 Not tainted 5.8.0-rc2+ #28
...
[ 2230.898175] WARNING: CPU: 3 PID: 1407 at drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:6265 mlxsw_sp_router_fib_event+0xfad/0x13e0
[ 2230.898179] Kernel panic - not syncing: panic_on_warn set ...
[ 2230.898183] CPU: 3 PID: 1407 Comm: syz-executor.0 Not tainted 5.8.0-rc2+ #28
[ 2230.898190] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014

Fixes: 3057224e

 ("mlxsw: spectrum_router: Implement FIB offload in deferred work")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>