cgroup: Make the offline drain interruptible

cgroup_subtree_control_write() and cgroup_type_write() drain dying csses
from the subtree before re-applying control, so that a controller being
re-enabled gets a fresh css instead of adopting one that is still dying. The
drain waits for offline_css() in TASK_UNINTERRUPTIBLE.

Since 1dffd95575eb ("cgroup: Defer kill_css_finish() in
cgroup_apply_control_disable()"), a disabled css offlines only after every
task that still pins it has finished exiting. The root cause of the hang is
an indefinite wait in a place where indefinite waits should not happen: a
task past exit_signals() can block in its exit path for as long as, say, a
FUSE daemon takes to answer a flush, and nothing can interrupt it because
prepare_signal() drops every signal for an exiting group. The drain then
escalated that into an uninterruptible wait for the writer, which sits in D
state for as long as the exiting task does. That is the hang syzbot reports
as "task hung in cgroup_subtree_control_write".

Make the drain interruptible. Nothing has been modified when it runs, so the
write restarts or fails with EINTR without side effects. cgroup1 mount and
remount drain the same way and become interruptible too.
cgroup_destroy_root() drains from a workqueue where no signal is ever
pending and keeps waiting as before.

This only stops the escalation. The write still waits for the exiting tasks,
and the indefinite wait in the exit path remains a separate problem.

Fixes: 1dffd95575eb ("cgroup: Defer kill_css_finish() in cgroup_apply_control_disable()")
Reported-by: syzbot+bb2e19a1190a556c01b1@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/6a23a4b4.e4db5ad2.3b7dfb.0000.GAE@google.com/
Signed-off-by: Tejun Heo <tj@kernel.org>