cifsd: check return value of ksmbd_vfs_getcasexattr() correctly

If ksmbd_vfs_getcasexattr() returns -ENOMEM, stream_buf is NULL,
it will cause null-ptr-deref when using it to copy memory. So we
need check the return value of ksmbd_vfs_getcasexattr() by comparing
with 0.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>