shared/298: Wire btrfs support in get_free_sectors

shared/298: Wire btrfs support in get_free_sectors

Add support for btrfs in shared/298. Achieve this by introducing 2
new awk scripts that parse relevant btrfs structures and print holes.
Additionally modify the test to create larger - 3gb filesystem in the
case of btrfs. This is needed so that distinct block groups are used
for data and metadata.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>