cifs: Fix prepare_write to negotiate wsize if needed

Fix cifs_prepare_write() to negotiate the wsize if it is unset.

Reviewed-by: Shyam Prasad N <nspmangalore@gmail.com>
Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>