smb/server: fix return value of smb2_open()

In most error cases, error code is not returned in smb2_open(),
__process_request() will not print error message.

Fix this by returning the correct value at the end of smb2_open().

Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>