wrapper: fix missing unlock() in cgroup_add_all_controllers()

wrapper: fix missing unlock() in cgroup_add_all_controllers()

Fix missing unlock(), reported by Coverity tool:

CID 313906 (#1 of 1): Missing unlock (LOCK)6. missing_unlock: Returning
without unlocking cg_mount_table_lock

add the missing pthread_rwlock_unlock() in the error path of the
cgroup_add_all_controllers()

Fixes: 4124f4d6853a ("wrapper: Add cgroup v2 support to cgroup_add_all_controllers()")
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>