cred: Fix RCU warnings in override/revert_creds

Fix RCU warnings in override_creds and revert_creds by turning
the RCU pointer into a normal pointer using rcu_replace_pointer.

These warnings were previously private to the cred code, but due
to the move into the header file they are now polluting unrelated
subsystems.

Fixes: 49dffdfd ("cred: Add a light version of override/revert_creds()")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lore.kernel.org/r/Z8QGQGW0IaSklKG7@gondor.apana.org.au


Signed-off-by: Christian Brauner <brauner@kernel.org>