fstests: btrfs: add a new test case for single data RAID56

fstests: btrfs: add a new test case for single data RAID56

Although single-data-RAID56, aka 2 disks raid5 or 3 disks raid6, is
not recommended for btrfs, btrfs still supports it as there are cases
like degraded writes can still lead to such cases.

But we do not have any proper tests for such corner cases.

Furthermore, RAID56 lib is going to drop the support of
single-data-RAID56, and btrfs will have its own handling of such cases,
thus coverage on single-data-RAID56 is required.

This new test case will have the following workload to verify the
behavior:

- Create a fs with single-data-RAID56

- Populate the fs with fsx and fsstress
  Fsx and fsstress will utilize read and write paths of RAID56.

- Mount each device in degraded mode and run read-only scrub on them
  Such scrub will utilize the following path of RAID56:
  * Regular data read
  * Reconstruction read
  * P/Q stripe scrub

- Re-scan all devices and add a new disk to the array
- Balance all data and scrub again
  This should utilized non-single-data-RAID56 handling.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>