x86/fpu: Split the state handling in fpu__drop()

Prepare fpu__drop() to use fpu->fpregs_active.

There are two distinct usecases for fpu__drop() in this context:
exit_thread() when called for 'current' in exit(), and when called
for another task in fork().

This patch does not change behavior, it only adds a couple of
debug checks and structures the code to make the ->fpregs_active
change more obviously correct.

All the complications will be removed later on.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20170923130016.21448-18-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/core.c

@@ -414,12 +414,19 @@ void fpu__drop(struct fpu *fpu)
 {
 	preempt_disable();
 
-	if (fpu->fpregs_active) {
-		/* Ignore delayed exceptions from user space */
-		asm volatile("1: fwait\n"
-			     "2:\n"
-			     _ASM_EXTABLE(1b, 2b));
-		fpregs_deactivate(fpu);
+	if (fpu == &current->thread.fpu) {
+		WARN_ON_FPU(fpu->fpstate_active != fpu->fpregs_active);
+
+		if (fpu->fpregs_active) {
+			/* Ignore delayed exceptions from user space */
+			asm volatile("1: fwait\n"
+				     "2:\n"
+				     _ASM_EXTABLE(1b, 2b));
+			if (fpu->fpregs_active)
+				fpregs_deactivate(fpu);
+		}
+	} else {
+		WARN_ON_FPU(fpu->fpregs_active);
 	}
 
 	fpu->fpstate_active = 0;