x86/fpu: Optimize fpu__save()

So fpu__save() does this currently:

		copy_fpregs_to_fpstate(fpu);
		if (!use_eager_fpu())
			fpregs_deactivate(fpu);

... which deactivates the FPU on lazy switching systems unconditionally.

Both usecases of fpu__save() use this function to save the
FPU state into a fpstate: fork()/clone() and math error signal handling.

The unconditional disabling of FPU registers in the lazy switching
case is probably a mistaken conversion of old FNSAVE code (that had
to disable FPU registers).

So speed up this code by only disabling FPU registers when absolutely
necessary: when indicated by the copy_fpregs_to_fpstate() return
code:

		if (!copy_fpregs_to_fpstate(fpu))
			fpregs_deactivate(fpu);
Reviewed-by: NBorislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/core.c

@@ -170,7 +170,7 @@ void irq_ts_restore(int TS_state)
 EXPORT_SYMBOL_GPL(irq_ts_restore);
 
 /*
- * Save the FPU state (initialize it if necessary):
+ * Save the FPU state (mark it for reload if necessary):
  *
  * This only ever gets called for the current task.
  */
@@ -180,8 +180,7 @@ void fpu__save(struct fpu *fpu)
 
 	preempt_disable();
 	if (fpu->fpregs_active) {
-		copy_fpregs_to_fpstate(fpu);
-		if (!use_eager_fpu())
+		if (!copy_fpregs_to_fpstate(fpu))
 			fpregs_deactivate(fpu);
 	}
 	preempt_enable();