generic: make 17[1-4] work well when btrfs compression is enabled

generic: make 17[1-4] work well when btrfs compression is enabled

When enabling btrfs compression, original codes can not fill fs
correctly, here we introduce _fill_fs() in common/rc, which'll keep
creating and writing files until enospc error occurs. Note _fill_fs
is copied from tests/generic/256, but with some minor modifications.

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>