Modifying a MCA bank's MCA_CTL bits which control which error types to
be reported is done over

  /sys/devices/system/machinecheck/
  ├── machinecheck0
  │   ├── bank0
  │   ├── bank1
  │   ├── bank10
  │   ├── bank11
  ...

sysfs nodes by writing the new bit mask of events to enable.

When the write is accepted, the kernel deletes all current timers and
reinits all banks.

Doing that in parallel can lead to initializing a timer which is already
armed and in the timer wheel, i.e., in use already:

  ODEBUG: init active (active state 0) object: ffff888063a28000 object
  type: timer_list hint: mce_timer_fn+0x0/0x240 arch/x86/kernel/cpu/mce/core.c:2642
  WARNING: CPU: 0 PID: 8120 at lib/debugobjects.c:514
  debug_print_object+0x1a0/0x2a0 lib/debugobjects.c:514

Fix that by grabbing the sysfs mutex as the rest of the MCA sysfs code
does.

Reported by: Yue Sun <samsun1006219@gmail.com>
Reported by: xingwei lee <xrivendell7@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/CAEkJfYNiENwQY8yV1LYJ9LjJs%2Bx_-PqMv98gKig55=2vbzffRw@mail.gmail.com