s4-backupkey: Set defined cert serialnumber

s4-backupkey: Set defined cert serialnumber

[MS-BKRP] 2.2.1 specifies that the serialnumber of the certificate
should be set identical to the subjectUniqueID. In fact certificates
generated by native AD have this field encoded in little-endian format.
See also
https://www.mail-archive.com/cifs-protocol@cifs.org/msg01364.html

Signed-off-by: Arvid Requate <requate@univention.de>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>