fsadm: better check for getsize64 support

fsadm: better check for getsize64 support

Older blockdev tool return failure error code with --help,
and since now the tool abort on command failure, lets
detect missing --getsize64 support directly by running
command and check if it returns something usable.

It's likely very hard to have the system with
such old blockdev tool and newer lvm2 compiled.