Fix the integer type in zfs_ioc_userspace_many()

Fix the mismatched type in zfs_ioc_userspace_many() and limit the
number of entries returned to 1000.  When a size larger than this
is requested the response is truncated, zfs_userspace() already
correctly handles short responses.  Historically, zfs_userspace()
has requested 100 entries at a time, this cap allows for 10x larger
batch sizes if needed in the future.

Reported-by: Yuxiang Yang, Yizhou Zhao, Ao Wang, Xuewei Feng, Qi Li,
Reported-by: and Ke Xu from Tsinghua University using GLM-5.1 from Z.ai
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #18615

module/zfs/zfs_ioctl.c

@@ -6652,21 +6652,27 @@ zfs_ioc_userspace_one(zfs_cmd_t *zc)
  * outputs:
  * zc_nvlist_dst[_size]	data buffer (array of zfs_useracct_t)
  * zc_cookie	zap cursor
+ *
+ * The zc_nvlist_dst output array is limited to 1000 entries.
  */
 static int
 zfs_ioc_userspace_many(zfs_cmd_t *zc)
 {
+	const size_t batch_limit = 1000 * sizeof (zfs_useracct_t);
+	uint64_t bufsize = MIN(zc->zc_nvlist_dst_size, batch_limit);
 	zfsvfs_t *zfsvfs;
-	int bufsize = zc->zc_nvlist_dst_size;
 
-	if (bufsize <= 0)
+	if (bufsize < sizeof (zfs_useracct_t)) {
+		zc->zc_nvlist_dst_size = sizeof (zfs_useracct_t);
 		return (SET_ERROR(ENOMEM));
+	}
 
 	int error = zfsvfs_hold(zc->zc_name, FTAG, &zfsvfs, B_FALSE);
 	if (error != 0)
 		return (error);
 
 	void *buf = vmem_alloc(bufsize, KM_SLEEP);
+	zc->zc_nvlist_dst_size = bufsize;
 
 	error = zfs_userspace_many(zfsvfs, zc->zc_objset_type, &zc->zc_cookie,
 	    buf, &zc->zc_nvlist_dst_size, &zc->zc_guid);