common/rc: Fix check for SCRATCH_DEV_POOL presence in _scratch_dev_pool_get

common/rc: Fix check for SCRATCH_DEV_POOL presence in _scratch_dev_pool_get

Current check is buggy because it can never trigger as even if
SCRATCH_DEV_POOL is not defined config_ndevs will get a value of 0
from 'wc -w', this in turn makes 'typeset -p config_ndevs' always
return 0, triggering the existing check a noop.

Fix this by explicitly checking for the presence of SCHRATC_DEV_POOL

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>