smb3: fix posix extensions mount option

We were incorrectly initializing the posix extensions in the
conversion to the new mount API.

CC: <stable@vger.kernel.org> # 5.11+
Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>