SELinux: make it harder to get the number of mnt opts wrong

Instead of just hard coding a value, use the enum to out benefit.
Signed-off-by: Eric Paris <eparis@redhat.com>

security/selinux/hooks.c

@@ -94,8 +94,6 @@
 #include "audit.h"
 #include "avc_ss.h"
 
-#define NUM_SEL_MNT_OPTS 5
-
 extern struct security_operations *security_ops;
 
 /* SECMARK reference count */
@@ -307,8 +305,11 @@ enum {
 	Opt_defcontext = 3,
 	Opt_rootcontext = 4,
 	Opt_labelsupport = 5,
+	Opt_nextmntopt = 6,
 };
 
+#define NUM_SEL_MNT_OPTS	(Opt_nextmntopt - 1)
+
 static const match_table_t tokens = {
 	{Opt_context, CONTEXT_STR "%s"},
 	{Opt_fscontext, FSCONTEXT_STR "%s"},