security/keys: fix missed RCU read section on lookup

security/keys: fix missed RCU read section on lookup

Nicholas Carlini reports that the keyring code calls assoc_array_find()
in find_key_to_update() without holding the RCU read lock, while the
assoc_array_gc() code really is designed around removing the node from
the tree and then freeing it after an RCU grace-period.

The regular key handling doesn't see this because holding the keyring
semaphore hides any lifetime issues, but the persistent key handling
uses a different model.

Instead of extending the keyring locking, just do the simple RCU locking
that the assoc_array was designed for.

Reported-by: Nicholas Carlini <npc@anthropic.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris James Morris <jmorris@namei.org>
Cc: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>