keyring: Remove now-redundant smp_read_barrier_depends()

Now that the associative-array library properly heads dependency chains,
the various smp_read_barrier_depends() calls in security/keys/keyring.c
are no longer needed.  This commit therefore removes them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: <keyrings@vger.kernel.org>
Cc: <linux-security-module@vger.kernel.org>
Reviewed-by: James Morris <james.l.morris@oracle.com>