sk-unix: fix e_str leak in unix_sk_id_add

coverity CID 389191:

int unix_sk_id_add(unsigned int ino)
2327{
2328        char *e_str;
2329
    1. alloc_fn: Storage is returned from allocation function malloc.
    2. var_assign: Assigning: ___p = storage returned from malloc(20UL).
    3. Condition !___p, taking false branch.
    4. leaked_storage: Variable ___p going out of scope leaks the storage it points to.
    5. var_assign: Assigning: e_str = ({...; ___p;}).
2330        e_str = xmalloc(20);
    6. Condition !e_str, taking false branch.
2331        if (!e_str)
2332                return -1;
    7. noescape: Resource e_str is not freed or pointed-to in snprintf.
2333        snprintf(e_str, 20, "unix[%u]", ino);
    8. noescape: Resource e_str is not freed or pointed-to in add_external. [show details]
    CID 389191 (#1 of 1): Resource leak (RESOURCE_LEAK)9. leaked_storage: Variable e_str going out of scope leaks the storage it points to.
2334        return add_external(e_str);
2335}

We should free e_str string after we finish it's use in unix_sk_id_add,
easiest way to do it is to use cleanup_free attribute.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

criu/sk-unix.c

@@ -2325,7 +2325,7 @@ static void try_resolve_unix_peer(struct unix_sk_info *ui)
 
 int unix_sk_id_add(unsigned int ino)
 {
-	char *e_str;
+	cleanup_free char *e_str = NULL;
 
 	e_str = xmalloc(20);
 	if (!e_str)