btrfs/150: add _scratch_dev_pool_get/put to run the test as expected

btrfs/150: add _scratch_dev_pool_get/put to run the test as expected

btrfs/150 uses RAID1 profile and make SCRATCH_DEV fail for test.
However, if SCRATCH_DEV_POOL consists more than two devices,
SCRATCH_DEV may not be used for RAID1 pair and the tests may not run
as expected.

Fix this by add _scratch_dev_pool_get/put like other tests (141, 143
etc.) do.

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>