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.comSigned-off-by: Eric W. Biederman <ebiederm@xmission.com>

arch/x86/kernel/vm86_32.c

@@ -106,10 +106,8 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
 	 */
 	local_irq_enable();
 
-	if (!vm86 || !vm86->user_vm86) {
-		pr_alert("no user_vm86: BAD\n");
-		do_exit(SIGSEGV);
-	}
+	BUG_ON(!vm86 || !vm86->user_vm86);
+
 	set_flags(regs->pt.flags, VEFLAGS, X86_EFLAGS_VIF | vm86->veflags_mask);
 	user = vm86->user_vm86;