service: cgroup.kill on instance teardown breaks restart of the servi…

[Aukansh]

Since commit f4d512d93a00a701c9604376e759ec60960d4dfb ("jail: place container init and exec into the target cgroup via clone3"), restarting a service that provides the ubus service object fails, and the service disappears from procd's service tree. On OpenWrt this is easily observable on rpcd.

instance_remove_cgroup() is called from instance_delete() while procd
is still processing a "service delete" ubus call issued by the service's
own init script (e.g. "/etc/init.d/rpcd restart" -> "stop" -> ubus call
service delete). Writing "1" to cgroup.kill at that point kills every
task in the cgroup, including the shell running the init script right
now. The shell dies before it can proceed to the "start" phase, the
service is left torn down, its name disappears from the services tree,
and a subsequent restart fails with:

Command failed: ubus call service delete { "name": "rpcd" } (Not found)

Fix this by simply attempting to remove the cgroup and ignoring EBUSY:
if the cgroup still has tasks, it is left in place and reused by the
next instance_start(); the kernel retires it once its last task exits.