KEYS: Fix RCU handling in key_gc_keyring()

key_gc_keyring() needs to either hold the RCU read lock or hold the keyring
semaphore if it's going to scan the keyring's list.  Given that it only needs
to read the key list, and it's doing so under a spinlock, the RCU read lock is
the thing to use.

Furthermore, the RCU check added in e7b0a61b is
incorrect as holding the spinlock on key_serial_lock is not grounds for
assuming a keyring's pointer list can be read safely.  Instead, a simple
rcu_dereference() inside of the previously mentioned RCU read lock is what we
want.

Reported-by: Serge E. Hallyn <serue@us.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>