rhashtable: avoid -Wrestrict warning on overlapping sprintf output

sprintf() is declared with a restrict keyword to not allow input and
output to point to the same buffer:

lib/test_rhashtable.c: In function 'print_ht':
lib/test_rhashtable.c:504:4: error: 'sprintf' argument 3 overlaps destination object 'buff' [-Werror=restrict]
  504 |    sprintf(buff, "%s\nbucket[%d] -> ", buff, i);
      |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
lib/test_rhashtable.c:489:7: note: destination object referenced by 'restrict'-qualified argument 1 was declared here
  489 |  char buff[512] = "";
      |       ^~~~

Rework this function to remember the last offset instead to
avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/test_rhashtable.c

@@ -487,6 +487,7 @@ static unsigned int __init print_ht(struct rhltable *rhlt)
 	struct rhashtable *ht;
 	const struct bucket_table *tbl;
 	char buff[512] = "";
+	int offset = 0;
 	unsigned int i, cnt = 0;
 
 	ht = &rhlt->ht;
@@ -501,18 +502,18 @@ static unsigned int __init print_ht(struct rhltable *rhlt)
 		next = !rht_is_a_nulls(pos) ? rht_dereference(pos->next, ht) : NULL;
 
 		if (!rht_is_a_nulls(pos)) {
-			sprintf(buff, "%s\nbucket[%d] -> ", buff, i);
+			offset += sprintf(buff + offset, "\nbucket[%d] -> ", i);
 		}
 
 		while (!rht_is_a_nulls(pos)) {
 			struct rhlist_head *list = container_of(pos, struct rhlist_head, rhead);
-			sprintf(buff, "%s[[", buff);
+			offset += sprintf(buff + offset, "[[");
 			do {
 				pos = &list->rhead;
 				list = rht_dereference(list->next, ht);
 				p = rht_obj(ht, pos);
 
-				sprintf(buff, "%s val %d (tid=%d)%s", buff, p->value.id, p->value.tid,
+				offset += sprintf(buff + offset, " val %d (tid=%d)%s", p->value.id, p->value.tid,
 					list? ", " : " ");
 				cnt++;
 			} while (list);
@@ -521,7 +522,7 @@ static unsigned int __init print_ht(struct rhltable *rhlt)
 			next = !rht_is_a_nulls(pos) ?
 				rht_dereference(pos->next, ht) : NULL;
 
-			sprintf(buff, "%s]]%s", buff, !rht_is_a_nulls(pos) ? " -> " : "");
+			offset += sprintf(buff + offset, "]]%s", !rht_is_a_nulls(pos) ? " -> " : "");
 		}
 	}
 	printk(KERN_ERR "\n---- ht: ----%s\n-------------\n", buff);