net/mlx5: fw_tracer, return NULL on create error

net/mlx5: fw_tracer, return NULL on create error

Tracer creation can fail by returning either NULL or ERR_PTR.
The return value is stored without a check on the device, and users
treat ERR_PTR and NULL the same way.
This also causes a crash in the core dump logic, which is missing the
ERR_PTR check and ends up dereferencing it, as shown in the trace below.

Switch tracer creation to return NULL on failure only, so callers only
need a single NULL check.

  Internal error: Oops: 0000000096000006 [#1]  SMP
  Modules linked in: mlx5_ib ib_uverbs ib_core ipv6 mlx5_core
  CPU: 1 UID: 0 PID: 12 Comm: kworker/u16:0 Not tainted 6.19.7 #1 PREEMPT(none)
  Workqueue: mlx5_health0001:01:00.0 mlx5_fw_reporter_err_work [mlx5_core]
  pstate: a3400009 (NzCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
  pc : mlx5_fw_tracer_trigger_core_dump_general+0x58/0xe0 [mlx5_core]
  lr : mlx5_fw_tracer_trigger_core_dump_general+0x40/0xe0 [mlx5_core]
  sp : ffff800081cf3c40
  x29: ffff800081cf3c90 x28: 0000000000000000 x27: 0000000000000000
  x26: ffff000080018828 x25: 0000000000000000 x24: ffff000080304a05
  x23: ffff800081cf3d80 x22: ffff0000847e01a0 x21: 0000000000000000
  x20: ffff0000847e01a0 x19: ffffffffffffffa1 x18: ffff80008310bbf0
  x17: ffff800080119650 x16: ffff80008010df54 x15: ffff80008010d4ac
  x14: ffff800079c202e4 x13: ffff80008002fe60 x12: ffff800080119650
  x11: ffff80008010df54 x10: ffff80008010d4ac x9 : ffff800079c203d8
  x8 : ffff800081cf3c88 x7 : 0000000000000000 x6 : 0000000000000000
  x5 : 0000000000000000 x4 : 0000000000000008 x3 : 0000000000000030
  x2 : 0000000000000008 x1 : 0000000000000000 x0 : 00000000c5c4000e
  Call trace:
   mlx5_fw_tracer_trigger_core_dump_general+0x58/0xe0 [mlx5_core] (P)
   mlx5_fw_reporter_dump+0x30/0x2e0 [mlx5_core]
   devlink_health_do_dump+0x9c/0x160
   devlink_health_report+0x1c0/0x288
   mlx5_fw_reporter_err_work+0xac/0xc0 [mlx5_core]
   process_one_work+0x15c/0x3d8
   worker_thread+0x18c/0x320
   kthread+0x148/0x228
   ret_from_fork+0x10/0x20
  Code: b9400000 5ac00800 7a401800 540003ca (3940a260)
  ---[ end trace 0000000000000000 ]---
  Kernel panic - not syncing: Oops: Fatal exception
  SMP: stopping secondary CPUs
  Kernel Offset: disabled
  CPU features: 0x000000,00078031,75fce5a1,35fffe67
  Memory Limit: none
  ---[ end Kernel panic - not syncing: Oops: Fatal exception ]---

Fixes: fd1483fe1f9f ("net/mlx5: Add support for FW reporter dump")
Signed-off-by: Michael Guralnik <michaelgur@nvidia.com>
Reviewed-by: Shay Drori <shayd@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260729080402.2427184-1-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>