fs: cifs: Remove unnecessary struct declaration

struct cifs_readdata is declared twice. One is declared
at 208th line.
And struct cifs_readdata is defined blew.
The declaration here is not needed. Remove the duplicate.
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsglob.h

@@ -1283,8 +1283,6 @@ struct cifs_aio_ctx {
 	bool			direct_io;
 };
 
-struct cifs_readdata;
-
 /* asynchronous read support */
 struct cifs_readdata {
 	struct kref			refcount;