src/config: Fix data race reported by Coverity

src/config: Fix data race reported by Coverity

Fix the following data race issue reported by Coverity:

CID 465888:Check of thread-shared field evades lock acquisition
(LOCK_EVASION):

"The data guarded by this critical section may be read while in an
inconsistent state or modified by multiple racing threads.

In cgroup_config_insert_into_namespace_table: Checking the value of a
thread-shared field outside of a locked region to determine if a locked
operation involving that thread shared field has completed."

Fix it by moving the namespace_table_index value check too under the
namespace_table_lock

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>