mount: fix e_str leak in ext_mount_add

coverity CID 389202:
54int ext_mount_add(char *key, char *val)
 55{
 56        char *e_str;
 57
   1. alloc_fn: Storage is returned from allocation function malloc.
   2. var_assign: Assigning: ___p = storage returned from malloc(strlen(key) + strlen(val) + 8UL).
   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;}).
 58        e_str = xmalloc(strlen(key) + strlen(val) + 8);
   6. Condition !e_str, taking false branch.
 59        if (!e_str)
 60                return -1;
...
   7. noescape: Resource e_str is not freed or pointed-to in sprintf.
 73        sprintf(e_str, "mnt[%s]:%s", key, val);
   8. noescape: Resource e_str is not freed or pointed-to in add_external. [show details]
   CID 389202 (#1 of 1): Resource leak (RESOURCE_LEAK)9. leaked_storage: Variable e_str going out of scope leaks the storage it points to.
 74        return add_external(e_str);
 75}

We need to free e_str after add_external used it.

v2: use cleanup_free attribute (@adrianreber)

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

criu/mount.c

@@ -55,7 +55,7 @@ static LIST_HEAD(delayed_unbindable);
 
 int ext_mount_add(char *key, char *val)
 {
-	char *e_str;
+	cleanup_free char *e_str = NULL;
 
 	e_str = xmalloc(strlen(key) + strlen(val) + 8);
 	if (!e_str)