ksmbd: fix uninitialized symbol 'pntsd_size'

No check for if "rc" is an error code for build_sec_desc().
This can cause problems with using uninitialized pntsd_size.

Fixes: e2f34481 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org # v5.15
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>