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 <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: H Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20211020174406.17889-9-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>