CIFS: Move open file handling to writepages

Currently we check for an open file existence in wdata_send_pages()
which doesn't provide an easy way to handle error codes that will
be returned from find_writable_filehandle() once it is changed.
Move the check to writepages.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>