x86/fpu: Introduce restore_init_xstate()

Extract the "use_eager_fpu()" code from drop_init_fpu() into a new,
simple helper restore_init_xstate(). The next patch adds another user.

- It is not clear why we do not check use_fxsr() like fpu_restore_checking()
  does. eager_fpu_init_bp() calls setup_init_fpu_buf() too, and we have the
  "eagerfpu=on" kernel option.

- Ignoring the fact that init_xstate_buf is "struct xsave_struct *", not
  "union thread_xstate *", it is not clear why we can not simply use
  fpu_restore_checking() and avoid the code duplication.

- It is not clear why we can't call setup_init_fpu_buf() unconditionally
  to always create init_xstate_buf(). Then do_device_not_available() path
  (at least) could use restore_init_xstate() too. It doesn't need to init
  fpu->state, its content doesn't matter until unlazy_fpu()/__switch_to()/etc
  which overwrites this memory anyway.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pekka Riikonen <priikone@iki.fi>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150311173429.GD5032@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/fpu-internal.h

@@ -401,16 +401,20 @@ static inline void drop_fpu(struct task_struct *tsk)
 	preempt_enable();
 }
 
+static inline void restore_init_xstate(void)
+{
+	if (use_xsave())
+		xrstor_state(init_xstate_buf, -1);
+	else
+		fxrstor_checking(&init_xstate_buf->i387);
+}
+
 static inline void drop_init_fpu(struct task_struct *tsk)
 {
 	if (!use_eager_fpu())
 		drop_fpu(tsk);
-	else {
-		if (use_xsave())
-			xrstor_state(init_xstate_buf, -1);
-		else
-			fxrstor_checking(&init_xstate_buf->i387);
-	}
+	else
+		restore_init_xstate();
 }
 
 /*