net/mlx5: Fix variable not being completed when function returns

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5: Fix variable not being completed when function returns

When cmd_alloc_index(), fails cmd_work_handler() needs

to complete ent->slotted before returning early.

Otherwise the task which issued the command may hang:

mlx5_core 0000:01:00.0: cmd_work_handler:877:(pid 3880418): failed to allocate command entry

INFO: task kworker/13:2:4055883 blocked for more than 120 seconds.

Not tainted 4.19.90-25.44.v2101.ky10.aarch64 #1

"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.

kworker/13:2 D 0 4055883 2 0x00000228

Workqueue: events mlx5e_tx_dim_work [mlx5_core]

Call trace:

__switch_to+0xe8/0x150

__schedule+0x2a8/0x9b8

schedule+0x2c/0x88

schedule_timeout+0x204/0x478

wait_for_common+0x154/0x250

wait_for_completion+0x28/0x38

cmd_exec+0x7a0/0xa00 [mlx5_core]

mlx5_cmd_exec+0x54/0x80 [mlx5_core]

mlx5_core_modify_cq+0x6c/0x80 [mlx5_core]

mlx5_core_modify_cq_moderation+0xa0/0xb8 [mlx5_core]

mlx5e_tx_dim_work+0x54/0x68 [mlx5_core]

process_one_work+0x1b0/0x448

worker_thread+0x54/0x468

kthread+0x134/0x138

ret_from_fork+0x10/0x18