btrfs: add functions to set and reset required number of SCRATCH_DEV_POOL

btrfs: add functions to set and reset required number of SCRATCH_DEV_POOL

This patch provides functions
 _scratch_dev_pool_get()
 _scratch_dev_pool_put()

Which will help to set/reset SCRATCH_DEV_POOL with the required
number of devices. SCRATCH_DEV_POOL_SAVED will hold all the devices.

Usage:
  _scratch_dev_pool_get() <ndevs>
  :: do stuff

  _scratch_dev_pool_put()

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>