nsfs: keep namespace tree fields stable until after RCU grace period

struct ns_common overlays struct ns_tree with the rcu_head used by kfree_rcu().
That lets the deferred-free machinery overwrite ns_id and __ns_ref_active as
soon as a namespace is queued for freeing, even though nsfs tree walkers may
still read those fields under RCU after ns_tree_remove(). KASAN reports slab
UAF.

Keep the tree state and deferred-free callback storage separate. Namespace tree
readers can then continue to validate and take references until the grace
period has elapsed.

Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Link: https://patch.msgid.link/20260814102005.1939777-1-Jeremy.Jean@oss.cyber.gouv.fr
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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