RDMA/mlx5: Fix crash when unbind multiport slave

Fix the below crash when deleting a slave from the unaffiliated list
twice. First time when the slave is bound to the master and the second
when the slave is unloaded.

Fix it by checking if slave is unaffiliated (doesn't have ib device)
before removing from the list.

RIP: 0010:mlx5r_mp_remove+0x4e/0xa0 [mlx5_ib]
Call Trace:
auxiliary_bus_remove+0x18/0x30
__device_release_driver+0x177/x220
device_release_driver+0x24/0x30
bus_remove_device+0xd8/0x140
device_del+0x18a/0x3e0
mlx5_rescan_drivers_locked+0xa9/0x210 [mlx5_core]
mlx5_unregister_device+0x34/0x60 [mlx5_core]
mlx5_uninit_one+0x32/0x100 [mlx5_core]
remove_one+0x6e/0xe0 [mlx5_core]
pci_device_remove+0x36/0xa0
__device_release_driver+0x177/0x220
device_driver_detach+0x3c/0xa0
unbind_store+0x113/0x130
kernfs_fop_write_iter+0x110/0x1a0
new_sync_write+0x116/0x1a0
vfs_write+0x1ba/0x260
ksys_write+0x5f/0xe0
do_syscall_64+0x3d/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 93f8244431ad ("RDMA/mlx5: Convert mlx5_ib to use auxiliary bus")
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Maor Gottlieb <[e-mail]>
Signed-off-by: Leon Romanovsky <[e-mail]>
Signed-off-by: Jason Gunthorpe <[e-mail]>

drivers/infiniband/hw/mlx5/main.c

···
 	mutex_lock(&mlx5_ib_multiport_mutex);
 	if (mpi->ibdev)
 		mlx5_ib_unbind_slave_port(mpi->ibdev, mpi);
-	list_del(&mpi->list);
+	else
+		list_del(&mpi->list);
 	mutex_unlock(&mlx5_ib_multiport_mutex);
 	kfree(mpi);
 }