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>