smb: client: fix uninitialized variable in smb2_writev_callback

compiling with W=2 pointed out that "written may be used uninitialized"

Fixes: 20d72b00ca81 ("netfs: Fix the request's work item to not require a ref")
Cc: stable@vger.kernel.org
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>