getcifsacl: don't freely cast between wbcDomainSid and cifs_sid

getcifsacl: don't freely cast between wbcDomainSid and cifs_sid

While they are very similar, the cifs_sid is "packed" and the
wbcDomainSid isn't. There are also aliasing problems with gcc in
some cases. Instead of trying to cast and fix endianness in place,
make a separate copy instead.

Signed-off-by: Jeff Layton <jlayton@samba.org>