CVE-2018-16857 dsdb/util: Correctly treat lockOutObservationWindow as 64-bit int

CVE-2018-16857 dsdb/util: Correctly treat lockOutObservationWindow as 64-bit int

Commit 442a38c918ae1666b35 refactored some code into a new
get_lockout_observation_window() function. However, in moving the code,
an ldb_msg_find_attr_as_int64() inadvertently got converted to a
ldb_msg_find_attr_as_int().

ldb_msg_find_attr_as_int() will only work for values up to -2147483648
(about 3.5 minutes in MS timestamp form). Unfortunately, the automated
tests used a low enough timeout that they still worked, however,
password lockout would not work with the Samba default settings.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13683

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>