RDMA/mlx5: Don't access NULL-cleared mpi pointer

The "dev->port[i].mp.mpi" is set to NULL during mlx5_ib_unbind_slave_port()
execution, however that field is needed to add device to unaffiliated list.

Such flow causes to the following kernel panic while unloading mlx5_ib
module in multi-port mode, hence the device should be added to the list
prior to unbind call.

 RPC: Unregistered rdma transport module.
 RPC: Unregistered rdma backchannel transport module.
 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 0 P4D 0
 Oops: 0002 [#1] SMP NOPTI
 CPU: 4 PID: 1904 Comm: modprobe Not tainted 5.13.0-rc7_for_upstream_min_debug_2021_06_24_12_08 #1
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
 RIP: 0010:mlx5_ib_cleanup_multiport_master+0x18b/0x2d0 [mlx5_ib]
 Code: 00 04 0f 85 c4 00 00 00 48 89 df e8 ef fa ff ff 48 8b 83 40 0d 00 00 48 8b 15 b9 e8 05 00 4a 8b 44 28 20 48 89 05 ad e8 05 00 <48> c7 00 d0 57 c5 a0 48 89 50 08 48 89 02 39 ab 88 0a 00 00 0f 86
 RSP: 0018:ffff888116ee3df8 EFLAGS: 00010296
 RAX: 0000000000000000 RBX: ffff8881154f6000 RCX: 0000000000000080
 RDX: ffffffffa0c557d0 RSI: ffff88810b69d200 RDI: 000000000002d8a0
 RBP: 0000000000000002 R08: ffff888110780408 R09: 0000000000000000
 R10: ffff88812452e1c0 R11: fffffffffff7e028 R12: 0000000000000000
 R13: 0000000000000080 R14: ffff888102c58000 R15: 0000000000000000
 FS:  00007f884393a740(0000) GS:ffff8882f5a00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 00000001249f6004 CR4: 0000000000370ea0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 Call Trace:
  mlx5_ib_stage_init_cleanup+0x16/0xd0 [mlx5_ib]
  __mlx5_ib_remove+0x33/0x90 [mlx5_ib]
  mlx5r_remove+0x22/0x30 [mlx5_ib]
  auxiliary_bus_remove+0x18/0x30
  __device_release_driver+0x177/0x220
  driver_detach+0xc4/0x100
  bus_remove_driver+0x58/0xd0
  auxiliary_driver_unregister+0x12/0x20
  mlx5_ib_cleanup+0x13/0x897 [mlx5_ib]
  __x64_sys_delete_module+0x154/0x230
  ? exit_to_user_mode_prepare+0x104/0x140
  do_syscall_64+0x3f/0x80
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7f8842e095c7
 Code: 73 01 c3 48 8b 0d d9 48 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d a9 48 2c 00 f7 d8 64 89 01 48
 RSP: 002b:00007ffc68f6e758 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
 RAX: ffffffffffffffda RBX: 00005638207929c0 RCX: 00007f8842e095c7
 RDX: 0000000000000000 RSI: 0000000000000800 RDI: 0000563820792a28
 RBP: 00005638207929c0 R08: 00007ffc68f6d701 R09: 0000000000000000
 R10: 00007f8842e82880 R11: 0000000000000206 R12: 0000563820792a28
 R13: 0000000000000001 R14: 0000563820792a28 R15: 00007ffc68f6fb40
 Modules linked in: xt_MASQUERADE nf_conntrack_netlink nfnetlink iptable_nat xt_addrtype xt_conntrack nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 br_netfilter overlay rdma_ucm ib_iser libiscsi scsi_transport_iscsi rdma_cm iw_cm ib_ipoib ib_cm ib_umad mlx5_ib(-) mlx4_ib ib_uverbs ib_core mlx4_en mlx4_core mlx5_core ptp pps_core [last unloaded: rpcrdma]
 CR2: 0000000000000000
 ---[ end trace a0bb7e20804e9e9b ]---

Fixes: 7ce6095e ("RDMA/mlx5: Don't add slave port to unaffiliated list")
Link: https://lore.kernel.org/r/899ac1b33a995be5ec0e16a4765c4e43c2b1ba5b.1624956444.git.leonro@nvidia.com


Reviewed-by: Itay Aveksis <itayav@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>