smb3: must initialize two ACL struct fields to zero

Coverity spotted that we were not initalizing Stbz1 and Stbz2 to
zero in create_sd_buf.

Addresses-Coverity: 1513848 ("Uninitialized scalar variable")
Cc: <stable@vger.kernel.org>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>