net/mlx5: Fix flow counter bulk command out mailbox allocation

The FW command output length should be only the length of struct
mlx5_cmd_fc_bulk out field. Failing to do so will cause the memcpy
call which is invoked later in the driver to write over wrong memory
address and corrupt kernel memory which results in random crashes.

This bug was found using the kernel address sanitizer (kasan).

Fixes: a351a1b0

 ('net/mlx5: Introduce bulk reading of flow counters')
Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>