fstests: btrfs: Use compressible data

fstests: btrfs: Use compressible data

/dev/urandom is incompressible and, /dev/zero is highly compressible,
so both are less effective in testing the compress code logic in btrfs.

This patch introduces a text data generator
 cat /dev/urandom | od
to populate the files where /dev/urandom is currently being used in the
btrfs test cases.

And updates the _populate_fs() with a new option -c, so to instruct
to use the compressible data to populate the file(s).

[eguan: add comments, fix indention]

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>