aio: simplify KIOCB_KEY handling

No need to pass the key field to lookup_iocb to compare it with KIOCB_KEY,
as we can do that right after retrieving it from userspace.  Also move the
KIOCB_KEY definition to aio.c as it is an internal value not used by any
other place in the kernel.

Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/aio.h

@@ -8,8 +8,6 @@ struct kioctx;
 struct kiocb;
 struct mm_struct;
 
-#define KIOCB_KEY		0
-
 typedef int (kiocb_cancel_fn)(struct kiocb *);
 
 /* prototypes */