metadata: allow reading metadata with invalid creation_time

metadata: allow reading metadata with invalid creation_time

lvm2 till version 2.02.169 (commit 78d004efa8a1809cea68283e6204edfa9d7c1091)
was printing invalid creation_time argument into metadata on 32bit arch.

However with commit ba9820b14223b731125c83dbc9709aa44fdcdbf1 we started
to properly validate all input numbers and thus we refused to accept
invalid metadata with 'garbage' string - but this results in the
situation where metadata produced on older lvm2 on 32 bit architecture
will become unreadable after upgrade.

To fix this case - extend libdm parser in a way, that whenever we
find error integer value, we also check if the parsed value is not for
creation_time node and in this case we let the metadata pass through
with made-up date 2018-05-24 (release date of 2.02.169).