iotests: Fix 125 for growth_mode = metadata

iotests: Fix 125 for growth_mode = metadata

If we use growth_mode = metadata, it is very much possible that the file
uses more disk space after we have written something to the added area.
We did indeed want to test for this case, but unfortunately we evidently
just copied the code from the "Test creation preallocation" section and
forgot to replace "$create_mode" by "$growth_mode".

We never noticed because we only read the first number from qemu-img
info's "disk size" output -- and that is effectively useless, because
qemu-img prints a human-readable value (which generally includes a
decimal point).  That will be fixed in the patch after the next one.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190925183231.11196-2-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>