apparmor: add get_dfa() fn

The dfa is currently setup to be shared (has the basis of refcounting)
but currently can't be because the count can't be increased.

Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/include/match.h

@@ -127,6 +127,21 @@ unsigned int aa_dfa_next(struct aa_dfa *dfa, unsigned int state,
 
 void aa_dfa_free_kref(struct kref *kref);
 
+/**
+ * aa_get_dfa - increment refcount on dfa @p
+ * @dfa: dfa  (MAYBE NULL)
+ *
+ * Returns: pointer to @dfa if @dfa is NULL will return NULL
+ * Requires: @dfa must be held with valid refcount when called
+ */
+static inline struct aa_dfa *aa_get_dfa(struct aa_dfa *dfa)
+{
+	if (dfa)
+		kref_get(&(dfa->count));
+
+	return dfa;
+}
+
 /**
  * aa_put_dfa - put a dfa refcount
  * @dfa: dfa to put refcount   (MAYBE NULL)