net/mlx4_core: Add missed mlx4_free_cmd_mailbox()

mlx4_do_mirror_rule() forgets to call mlx4_free_cmd_mailbox() to
free the memory region allocated by mlx4_alloc_cmd_mailbox() before
an exit.
Add the missed call to fix it.

Fixes: 78efed27

 ("net/mlx4_core: Support mirroring VF DMFS rules on both ports")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20210221143559.390277-1-hslester96@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>