cifs: fix CIFS_IOC_GET_MNT_INFO oops

An open directory may have a NULL private_data pointer prior to readdir.

Fixes: 0de1f4c6 ("Add way to query server fs info for smb3")
Cc: stable@vger.kernel.org
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Steve French <smfrench@gmail.com>