net/mlx5: LAG, fix logic over MLX5_LAG_FLAG_NDEVS_READY

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

net/mlx5: LAG, fix logic over MLX5_LAG_FLAG_NDEVS_READY

Only set MLX5_LAG_FLAG_NDEVS_READY if both netdevices are registered.
Doing so guarantees that both ldev->pf[MLX5_LAG_P0].dev and
ldev->pf[MLX5_LAG_P1].dev have valid pointers when
MLX5_LAG_FLAG_NDEVS_READY is set.

The core issue is asymmetry in setting MLX5_LAG_FLAG_NDEVS_READY and
clearing it. Setting it is done wrongly when both
ldev->pf[MLX5_LAG_P0].dev and ldev->pf[MLX5_LAG_P1].dev are set;
clearing it is done right when either of ldev->pf[i].netdev is cleared.

Consider the following scenario:
1. PF0 loads and sets ldev->pf[MLX5_LAG_P0].dev to a valid pointer
2. PF1 loads and sets both ldev->pf[MLX5_LAG_P1].dev and
ldev->pf[MLX5_LAG_P1].netdev with valid pointers. This results in
MLX5_LAG_FLAG_NDEVS_READY is set.
3. PF0 is unloaded before setting dev->pf[MLX5_LAG_P0].netdev.
MLX5_LAG_FLAG_NDEVS_READY remains set.

Further execution of mlx5_do_bond() will result in null pointer
dereference when calling mlx5_lag_is_multipath()

This patch fixes the following call trace actually encountered:

[ 1293.475195] BUG: kernel NULL pointer dereference, address: 00000000000009a8
[ 1293.478756] #PF: supervisor read access in kernel mode
[ 1293.481320] #PF: error_code(0x0000) - not-present page
[ 1293.483686] PGD 0 P4D 0
[ 1293.484434] Oops: 0000 [#1] SMP PTI
[ 1293.485377] CPU: 1 PID: 23690 Comm: kworker/u16:2 Not tainted 5.18.0-rc5_for_upstream_min_debug_2022_05_05_10_13 #1
[ 1293.488039] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
[ 1293.490836] Workqueue: mlx5_lag mlx5_do_bond_work [mlx5_core]
[ 1293.492448] RIP: 0010:mlx5_lag_is_multipath+0x5/0x50 [mlx5_core]
[ 1293.494044] Code: e8 70 40 ff e0 48 8b 14 24 48 83 05 5c 1a 1b 00 01 e9 19 ff ff ff 48 83 05 47 1a 1b 00 01 eb d7 0f 1f 44 00 00 0f 1f 44 00 00 <48> 8b 87 a8 09 00 00 48 85 c0 74 26 48 83 05 a7 1b 1b 00 01 41 b8
[ 1293.498673] RSP: 0018:ffff88811b2fbe40 EFLAGS: 00010202
[ 1293.500152] RAX: ffff88818a94e1c0 RBX: ffff888165eca6c0 RCX: 0000000000000000
[ 1293.501841] RDX: 0000000000000001 RSI: ffff88818a94e1c0 RDI: 0000000000000000
[ 1293.503585] RBP: 0000000000000000 R08: ffff888119886740 R09: ffff888165eca73c
[ 1293.505286] R10: 0000000000000018 R11: 0000000000000018 R12: ffff88818a94e1c0
[ 1293.506979] R13: ffff888112729800 R14: 0000000000000000 R15: ffff888112729858
[ 1293.508753] FS: 0000000000000000(0000) GS:ffff88852cc40000(0000) knlGS:0000000000000000
[ 1293.510782] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1293.512265] CR2: 00000000000009a8 CR3: 00000001032d4002 CR4: 0000000000370ea0
[ 1293.514001] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1293.515806] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400