mkfs01.sh: use df -P

For the same reason as it was in
commit deab0acd12a6 ("df01: use the POSIX output format").

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
1 file changed