CVE-2021-20251 s4-rpc_server: Use authsam_search_account() to find the user

This helps the bad password and audit log handling code as it
allows assumptions to be made about the attributes found in
the variable "msg", such as that DSDB_SEARCH_SHOW_EXTENDED_DN
was used.

This ensures we can re-search on the DN via the embedded GUID,
which in in turn rename-proof.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14611

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
