Added SCRATCH_DEV_POOL to specify multiple disks for the btrfs RAID

Added SCRATCH_DEV_POOL to specify multiple disks for the btrfs RAID

SCRATCH_DEV takes single disk as the scratch place for testing. New
SCRATCH_DEV_POOL can used to specify multiple disks for the scratch
btrfs filesystem.

Using SCRATCH_DEV and or SCRATCH_DEV_POOL will follow the following logic.
  btrfs FS OR any FS
    SCRATCH_DEV_POOL is unset and SCRATCH_DEV is set
     . test-case with _require_scratch_dev_pool will not run
     . test-case without _require_scratch_dev_pool will run

    SCRATCH_DEV_POOL is set and SCRATCH_DEV is unset
     . test-case with _require_scratch_dev_pool
         - runs only if FSTYP=btrfs
     . test-case without _require_scratch_dev_pool will run using first
       dev in the SCRATCH_DEV_POOL as a SCRATCH_DEV
         - if FSTYP=btrfs it includes SCRATCH_DEV_POOL disks to the FS
         - if FSTYP=non-btrfs SCRATCH_DEV_POOL is ignored

    SCRATCH_DEV_POOL is set and SCRATCH_DEV is set
     . reports error in the config

    SCRATCH_DEV_POOL is unset and SCRATCH_DEV is unset
     . no change

Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>