x86/vm86: Make sure the free_vm86(task) definition uses its parameter even in the !CONFIG_VM86 case

I ran into a case in new code where free_vm86(task) was the only
consumer of the 'task' variable:

  arch/x86/kernel/process.c: In function ‘exit_thread’:
  arch/x86/kernel/process.c:118:31: error: unused variable ‘t’ [-Werror=unused-variable]

Robustify the definition in the !CONFIG_VM86 case.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/ZgaFfyHMOdLHEKm+@gmail.com