net/mlx4_core: Fix memory leak while delete slave's resources

mlx4_delete_all_resources_for_slave in resource tracker should free all
memory allocated for a slave.
While releasing memory of fs_rule, it misses releasing memory of
fs_rule->mirr_mbox.

Fixes: 78efed27

 ('net/mlx4_core: Support mirroring VF DMFS rules on both ports')
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>