kernfs: Improve kernfs_drain() and always call on removal

__kernfs_remove() was skipping draining based on KERNFS_ACTIVATED - whether
the node has ever been activated since creation. Instead, update it to
always call kernfs_drain() which now drains or skips based on the precise
drain conditions. This ensures that the nodes will be deactivated and
drained regardless of their states.

This doesn't make meaningful difference now but will enable deactivating and
draining nodes dynamically by making removals safe when racing those
operations.

While at it, drop / update comments.

v2: Fix the inverted test on kernfs_should_drain_open_files() noted by
    Chengming. This was fixed by the next unrelated patch in the previous
    posting.

Cc: Chengming Zhou <zhouchengming@bytedance.com>
Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220828050440.734579-6-tj@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>