CRED: Holding a spinlock does not imply the holding of RCU read lock

Change the credentials documentation to make it clear that the RCU read lock
must be explicitly held when accessing credentials pointers in some other task
than current.  Holding a spinlock does not implicitly hold the RCU read lock.

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