AppArmor: Fix oops in policy unpack auditing

Post unpacking of policy a verification pass is made on x transition
indexes.  When this fails a call to audit_iface is made resulting in an
oops, because audit_iface is expecting a valid buffer position but
since the failure comes from post unpack verification there is none.

Make the position argument optional so that audit_iface can be called
from post unpack verification.

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

security/apparmor/policy_unpack.c

@@ -84,7 +84,7 @@ static void audit_cb(struct audit_buffer *ab, void *va)
  * @new: profile if it has been allocated (MAYBE NULL)
  * @name: name of the profile being manipulated (MAYBE NULL)
  * @info: any extra info about the failure (MAYBE NULL)
- * @e: buffer position info (NOT NULL)
+ * @e: buffer position info
  * @error: error code
  *
  * Returns: %0 or error
@@ -95,7 +95,8 @@ static int audit_iface(struct aa_profile *new, const char *name,
 	struct aa_profile *profile = __aa_current_profile();
 	struct common_audit_data sa;
 	COMMON_AUDIT_DATA_INIT(&sa, NONE);
-	sa.aad.iface.pos = e->pos - e->start;
+	if (e)
+		sa.aad.iface.pos = e->pos - e->start;
 	sa.aad.iface.target = new;
 	sa.aad.name = name;
 	sa.aad.info = info;