signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON

signal/vm86_32: Replace open coded BUG_ON with an actual BUG_ON

The function save_v86_state is only called when userspace was
operating in vm86 mode before entering the kernel.  Not having vm86
state in the task_struct should never happen.  So transform the hand
rolled BUG_ON into an actual BUG_ON to make it clear what is
happening.

Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: [email protected]
Cc: H Peter Anvin <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Eric W. Biederman <[email protected]>