fs: cifs: Remove repeated struct declaration

struct cifs_writedata is declared twice.
One is declared at 209th line.
And struct cifs_writedata is defined blew.
The declaration hear is not needed. Remove the duplicate.

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: Steve French <stfrench@microsoft.com>