cgroup: reduce cgroup_file_kn_lock hold time in cgroup_file_notify()

cgroup: reduce cgroup_file_kn_lock hold time in cgroup_file_notify()

cgroup_file_notify() calls kernfs_notify() while holding the global
cgroup_file_kn_lock.  kernfs_notify() does non-trivial work including
wake_up_interruptible() and acquisition of a second global spinlock
(kernfs_notify_lock), inflating the hold time.

Take a kernfs_get() reference under the lock and call kernfs_notify()
after dropping it, following the pattern from cgroup_file_show().

Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>