generic: do operations which doesn't take data space on full fs

generic: do operations which doesn't take data space on full fs

This test will test if we can still do the following operations when a
full is full:
- buffered write into unpopulated preallocated extent
- clone the untouched preallocated extent
- fsync
- no data loss if power loss happens after above fsync
Above operations should not fail, as they takes no extra data space.

Xfs passes the test, while btrfs fails at fsync and has data loss.
The fix for btrfs is:
"btrfs: Flush before reflinking any extent to prevent NOCOW write falling
 back to CoW without data reservation"

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>