mlxsw: spectrum_router: Fix use after free

In mlxsw_sp_router_fib4_add_info_destroy(), the fib_entry pointer is used
after it has been freed by mlxsw_sp_fib_entry_destroy(). Use a temporary
variable to fix this.

Fixes: 61c503f9

 ("mlxsw: spectrum_router: Implement fib4 add/del switchdev obj ops")
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>