btrfs: use $FILEFRAG_PROG instead of filefrag

$FILEFRAG_PROG has been defined in common/config, so
we could apply it.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
2 files changed