This patch fixes the following compilation error:
```
../../module/zfs/json_stats.c: In function ‘nvlist_to_json’:
../../module/zfs/json_stats.c:92:4: error: a label can only be part of a statement and a declaration is not a statement
uint64_t *u = (uint64_t *)p;
^~~~~~~~
../../module/zfs/json_stats.c:102:4: error: a label can only be part of a statement and a declaration is not a statement
nvlist_t **a = (nvlist_t **)p;
^~~~~~~~
```
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