cgroup: Make rebind_subsystems() disable v2 controllers all at once

cgroup: Make rebind_subsystems() disable v2 controllers all at once

It was found that the following warning was displayed when remounting
controllers from cgroup v2 to v1:

[ 8042.997778] WARNING: CPU: 88 PID: 80682 at kernel/cgroup/cgroup.c:3130 cgroup_apply_control_disable+0x158/0x190
   :
[ 8043.091109] RIP: 0010:cgroup_apply_control_disable+0x158/0x190
[ 8043.096946] Code: ff f6 45 54 01 74 39 48 8d 7d 10 48 c7 c6 e0 46 5a a4 e8 7b 67 33 00 e9 41 ff ff ff 49 8b 84 24 e8 01 00 00 0f b7 40 08 eb 95 <0f> 0b e9 5f ff ff ff 48 83 c4 08 5b 5d 41 5c 41 5d 41 5e 41 5f c3
[ 8043.115692] RSP: 0018:ffffba8a47c23d28 EFLAGS: 00010202
[ 8043.120916] RAX: 0000000000000036 RBX: ffffffffa624ce40 RCX: 000000000000181a
[ 8043.128047] RDX: ffffffffa63c43e0 RSI: ffffffffa63c43e0 RDI: ffff9d7284ee1000
[ 8043.135180] RBP: ffff9d72874c5800 R08: ffffffffa624b090 R09: 0000000000000004
[ 8043.142314] R10: ffffffffa624b080 R11: 0000000000002000 R12: ffff9d7284ee1000
[ 8043.149447] R13: ffff9d7284ee1000 R14: ffffffffa624ce70 R15: ffffffffa6269e20
[ 8043.156576] FS:  00007f7747cff740(0000) GS:ffff9d7a5fc00000(0000) knlGS:0000000000000000
[ 8043.164663] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 8043.170409] CR2: 00007f7747e96680 CR3: 0000000887d60001 CR4: 00000000007706e0
[ 8043.177539] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 8043.184673] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 8043.191804] PKRU: 55555554
[ 8043.194517] Call Trace:
[ 8043.196970]  rebind_subsystems+0x18c/0x470
[ 8043.201070]  cgroup_setup_root+0x16c/0x2f0
[ 8043.205177]  cgroup1_root_to_use+0x204/0x2a0
[ 8043.209456]  cgroup1_get_tree+0x3e/0x120
[ 8043.213384]  vfs_get_tree+0x22/0xb0
[ 8043.216883]  do_new_mount+0x176/0x2d0
[ 8043.220550]  __x64_sys_mount+0x103/0x140
[ 8043.224474]  do_syscall_64+0x38/0x90
[ 8043.228063]  entry_SYSCALL_64_after_hwframe+0x44/0xae

It was caused by the fact that rebind_subsystem() disables
controllers to be rebound one by one. If more than one disabled
controllers are originally from the default hierarchy, it means that
cgroup_apply_control_disable() will be called multiple times for the
same default hierarchy. A controller may be killed by css_kill() in
the first round. In the second round, the killed controller may not be
completely dead yet leading to the warning.

To avoid this problem, we collect all the ssid's of controllers that
needed to be disabled from the default hierarchy and then disable them
in one go instead of one by one.

Fixes: 334c3679ec4b ("cgroup: reimplement rebind_subsystems() using cgroup_apply_control() and friends")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>