btrfs/027: make use of new device get and put helper functions

btrfs/027: make use of new device get and put helper functions

Below patches added helper function to get the requested
number of devices for scratch and spare device

btrfs: add functions to get and put a device for replace target
btrfs: add functions to set and reset required number of SCRATCH_DEV_POOL

This patch makes use of them.

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