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: 49dffdf ("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>

Author: herbertx

include/linux/cred.h

@@ -172,18 +172,12 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred)
 
 static inline const struct cred *override_creds(const struct cred *override_cred)
 {
-	const struct cred *old = current->cred;
-
-	rcu_assign_pointer(current->cred, override_cred);
-	return old;
+	return rcu_replace_pointer(current->cred, override_cred, 1);
 }
 
 static inline const struct cred *revert_creds(const struct cred *revert_cred)
 {
-	const struct cred *override_cred = current->cred;
-
-	rcu_assign_pointer(current->cred, revert_cred);
-	return override_cred;
+	return rcu_replace_pointer(current->cred, revert_cred, 1);
 }
 
 /**