cifsd: handle unhashed dentry in ksmbd_vfs_mkdir

vfs_mkdir could return the dentry left unhashed negative on success.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>