mlx4: Fix a memory leak bug.

In function mlx4_opreq_action(), pointer "mailbox" is not released,
when mlx4_cmd_box() return and error, causing a memory leak bug.
Fix this issue by going to "out" label, mlx4_free_cmd_mailbox() can
free this pointer.

Linux commit:
febfd9d3c7f74063e8e630b15413ca91b567f963

PR: 264056
MFC after: 1 week
Sponsored by: NVIDIA Networking