x86/fpu: Optimize fpu__activate_fpstate_read()

fpu__activate_fpstate_read() is used before FPU registers are
read from the fpstate by ptrace and core dumping.

It's not necessary to unlazy non-current child tasks in this case,
since the reading of registers is non-destructive.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/core.c

@@ -312,10 +312,7 @@ void fpu__activate_fpstate_read(struct fpu *fpu)
 	if (fpu->fpregs_active) {
 		fpu__save(fpu);
 	} else {
-		if (fpu->fpstate_active) {
-			/* Invalidate any lazy state: */
-			fpu->last_cpu = -1;
-		} else {
+		if (!fpu->fpstate_active) {
 			fpstate_init(&fpu->state);
 
 			/* Safe to do for current and for stopped child tasks: */