dbcheck: Be more careful with link checks

dbcheck: Be more careful with link checks

Here we are more careful when checking links, flagging errors only
when a non-deleted forward link appears incorrect.  In particular, we
trust the GUID more than we trust the name, as otherwise we can get
caught out if there is a swap of names, (the link should follow the
swap, staying on the same target GUID).

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=12297