Fix zfs_clone_010_pos.ksh to verify the space used by multiple copies

The default blocksize in Linux is 1024 due to a GNU-ism. Setting the
expected blocksize resolves the issue. As mentioned in the PR an
alternate solution would be to set POSIXLY_CORRECT=1.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: yuxiang <guo.yong33@zte.com.cn>
Closes #5167