kernfs: Add KERNFS_REMOVING flags

KERNFS_ACTIVATED tracks whether a given node has ever been activated. As a
node was only deactivated on removal, this was used for

 1. Drain optimization (removed by the previous patch).
 2. To hide !activated nodes
 3. To avoid double activations
 4. Reject adding children to a node being removed
 5. Skip activaing a node which is being removed.

We want to decouple deactivation from removal so that nodes can be
deactivated and hidden dynamically, which makes KERNFS_ACTIVATED useless for
all of the above purposes.

#1 is already gone. #2 and #3 can instead test whether the node is currently
active. A new flag KERNFS_REMOVING is added to explicitly mark nodes which
are being removed for #4 and #5.

While this leaves KERNFS_ACTIVATED with no users, leave it be as it will be
used in a following patch.

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/20220828050440.734579-7-tj@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>