[PATCH] change io_cancel return code for no cancel case

Note that other than few exceptions, most of the current filesystem and/or
drivers do not have aio cancel specifically defined (kiob->ki_cancel field
is mostly NULL).  However, sys_io_cancel system call universally sets
return code to -EAGAIN.  This gives applications a wrong impression that
this call is implemented but just never works.  We have customer inquires
about this issue.

Changed by Benjamin LaHaise to EINVAL instead of ENOSYS
Signed-off-by: S. Wendy Cheng <wcheng@redhat.com>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/aio.c

@@ -1673,7 +1673,7 @@ asmlinkage long sys_io_cancel(aio_context_t ctx_id, struct iocb __user *iocb,
 				ret = -EFAULT;
 		}
 	} else
-		printk(KERN_DEBUG "iocb has no cancel operation\n");
+		ret = -EINVAL;
 
 	put_ioctx(ctx);