x86/kprobes: Fix build errors and blacklist context_track_user

This essentially reverts commit:

ecd50f714c42 ("kprobes, x86: Call exception_enter after kprobes handled")

since it causes build errors with CONFIG_CONTEXT_TRACKING and
that has been made from misunderstandings;
context_track_user_*() don't involve much in interrupt context,
it just returns if in_interrupt() is true.

Instead of changing the do_debug/int3(), this just adds
context_track_user_*() to kprobes blacklist, since those are
still can be called right before kprobes handles int3 and debug
exceptions, and probing those will cause an infinite loop.

Reported-by: Frederic Weisbecker <[e-mail]>
Signed-off-by: Masami Hiramatsu <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Jiri Kosina <[e-mail]>
Cc: Rusty Russell <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Seiji Aguchi <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Kees Cook <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/traps.c

···
 	if (poke_int3_handler(regs))
 		return;
 
+	prev_state = exception_enter();
 #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
 	if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
 				SIGTRAP) == NOTIFY_STOP)
···
 
 #ifdef CONFIG_KPROBES
 	if (kprobe_int3_handler(regs))
-		return;
+		goto exit;
 #endif
-	prev_state = exception_enter();
 
 	if (notify_die(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
 			SIGTRAP) == NOTIFY_STOP)
···
 	unsigned long dr6;
 	int si_code;
 
+	prev_state = exception_enter();
+
 	get_debugreg(dr6, 6);
 
 	/* Filter out all the reserved bits which are preset to 1 */
···
 	if (kprobe_debug_handler(regs))
 		goto exit;
 #endif
-	prev_state = exception_enter();
 
 	if (notify_die(DIE_DEBUG, "debug", regs, (long)&dr6, error_code,
 							SIGTRAP) == NOTIFY_STOP)

kernel/context_tracking.c

···
 #include <linux/sched.h>
 #include <linux/hardirq.h>
 #include <linux/export.h>
+#include <linux/kprobes.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/context_tracking.h>
···
 	}
 	local_irq_restore(flags);
 }
+NOKPROBE_SYMBOL(context_tracking_user_enter);
 
 #ifdef CONFIG_PREEMPT
 /**
···
 	}
 	local_irq_restore(flags);
 }
+NOKPROBE_SYMBOL(context_tracking_user_exit);
 
 /**
  * __context_tracking_task_switch - context switch the syscall callbacks