selinux: retain const qualifier on string literal in avtab_hash_eval()

The second parameter `tag` of avtab_hash_eval() is only used for
printing.  In policydb_index() it is called with a string literal:

    avtab_hash_eval(&p->te_avtab, "rules");
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: slight formatting tweak in description]
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/avtab.c

@@ -354,7 +354,7 @@ int avtab_alloc_dup(struct avtab *new, const struct avtab *orig)
 	return avtab_alloc_common(new, orig->nslot);
 }
 
-void avtab_hash_eval(struct avtab *h, char *tag)
+void avtab_hash_eval(struct avtab *h, const char *tag)
 {
 	int i, chain_len, slots_used, max_chain_len;
 	unsigned long long chain2_len_sum;

security/selinux/ss/avtab.h

@@ -92,7 +92,7 @@ int avtab_alloc(struct avtab *, u32);
 int avtab_alloc_dup(struct avtab *new, const struct avtab *orig);
 struct avtab_datum *avtab_search(struct avtab *h, const struct avtab_key *k);
 void avtab_destroy(struct avtab *h);
-void avtab_hash_eval(struct avtab *h, char *tag);
+void avtab_hash_eval(struct avtab *h, const char *tag);
 
 struct policydb;
 int avtab_read_item(struct avtab *a, void *fp, struct policydb *pol,