x86/asm/entry/64: Simplify looping around preempt_schedule_irq()

At the 'exit_intr' label we test whether interrupt/exception was in
kernel. If it did, we jump to the preemption check. If preemption
does happen (IOW if we call preempt_schedule_irq()), we go back to
'exit_intr'.

But it's pointless, we already know that the test succeeded last
time, preemption doesn't change the fact that interrupt/exception
was in the kernel.

We can go back directly to checking PER_CPU_VAR(__preempt_count) instead.

This makes the 'exit_intr' label unused, drop it.

Signed-off-by: Denys Vlasenko <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Will Drewry <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/entry_64.S

···
 	CFI_DEF_CFA_REGISTER	rsp
 	CFI_ADJUST_CFA_OFFSET	RBP
 
-exit_intr:
 	testl $3,CS(%rsp)
 	je retint_kernel
 	/* Interrupt came from user space */
···
 #ifdef CONFIG_PREEMPT
 	/* Interrupts are off */
 	/* Check if we need preemption */
-	cmpl	$0,PER_CPU_VAR(__preempt_count)
-	jnz	1f
 	bt	$9,EFLAGS(%rsp)	/* interrupts were off? */
 	jnc	1f
+0:	cmpl	$0,PER_CPU_VAR(__preempt_count)
+	jnz	1f
 	call	preempt_schedule_irq
-	jmp	exit_intr
+	jmp	0b
 1:
 #endif
 	/*