x86/fpu: Optimize fpu_copy()

Optimize fpu_copy() a bit by expanding the ->fpstate_active == 1
portion of fpu__save() into it.

( The main purpose of this change is to enable another, larger
  optimization that will be done in the next patch. )
Reviewed-by: Borislav 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: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/core.c

@@ -224,7 +224,10 @@ static void fpu_copy(struct fpu *dst_fpu, struct fpu *src_fpu)
 		memset(&dst_fpu->state.xsave, 0, xstate_size);
 		copy_fpregs_to_fpstate(dst_fpu);
 	} else {
-		fpu__save(src_fpu);
+		preempt_disable();
+		if (!copy_fpregs_to_fpstate(src_fpu))
+			fpregs_deactivate(src_fpu);
+		preempt_enable();
 		memcpy(&dst_fpu->state, &src_fpu->state, xstate_size);
 	}
 }