apparmor: document first entry is in packed perms struct is reserved

Add a comment to unpack_perm to document the first entry in the packed
perms struct is reserved, and make a non-functional change of unpacking
to a temporary stack variable named "reserved" to help suppor the
documentation of which value is reserved.

Suggested-by: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: jrjohansen

security/apparmor/policy_unpack.c

@@ -645,10 +645,13 @@ static bool unpack_rlimits(struct aa_ext *e, struct aa_ruleset *rules)
 
 static bool unpack_perm(struct aa_ext *e, u32 version, struct aa_perms *perm)
 {
+	u32 reserved;
+
 	if (version != 1)
 		return false;
 
-	return	aa_unpack_u32(e, &perm->allow, NULL) &&
+	/* reserved entry is for later expansion, discard for now */
+	return	aa_unpack_u32(e, &reserved, NULL) &&
 		aa_unpack_u32(e, &perm->allow, NULL) &&
 		aa_unpack_u32(e, &perm->deny, NULL) &&
 		aa_unpack_u32(e, &perm->subtree, NULL) &&