security: add a secctx_to_secid() hook

Add a secctx_to_secid() LSM hook to go along with the existing
secid_to_secctx() LSM hook.  This patch also includes the SELinux
implementation for this hook.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>

security/dummy.c

diff --git a/security/dummy.c b/security/dummy.c
index 8e34e03..48d4b0a 100644
--- a/security/dummy.c
+++ b/security/dummy.c
@@ -946,6 +946,11 @@
 	return -EOPNOTSUPP;
 }
 
+static int dummy_secctx_to_secid(char *secdata, u32 seclen, u32 *secid)
+{
+	return -EOPNOTSUPP;
+}
+
 static void dummy_release_secctx(char *secdata, u32 seclen)
 {
 }
@@ -1106,6 +1111,7 @@
  	set_to_dummy_if_null(ops, getprocattr);
  	set_to_dummy_if_null(ops, setprocattr);
  	set_to_dummy_if_null(ops, secid_to_secctx);
+	set_to_dummy_if_null(ops, secctx_to_secid);
  	set_to_dummy_if_null(ops, release_secctx);
 #ifdef CONFIG_SECURITY_NETWORK
 	set_to_dummy_if_null(ops, unix_stream_connect);