x86, fpu: check __clear_user() return value

fix warning:

  arch/x86/kernel/xsave.c: In function ‘save_i387_xstate’:
  arch/x86/kernel/xsave.c:98: warning: ignoring return value of ‘__clear_user’, declared with attribute warn_unused_result

check the return value and act on it. We should not be ignoring faults
at this point.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/xsave.c

@@ -95,7 +95,9 @@ int save_i387_xstate(void __user *buf)
 	 	 * Start with clearing the user buffer. This will present a
 	 	 * clean context for the bytes not touched by the fxsave/xsave.
 		 */
-		__clear_user(buf, sig_xstate_size);
+		err = __clear_user(buf, sig_xstate_size);
+		if (err)
+			return err;
 
 		if (task_thread_info(tsk)->status & TS_XSAVE)
 			err = xsave_user(buf);