dm thin: disable discard support for thin devices if pool's is disabled

If the pool is configured with 'ignore_discard' its discard support is
disabled.  The pool's thin devices should also have queue_limits that
reflect discards are disabled.

Fixes: 34fbcf6 ("dm thin: range discard support")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 4.1+

drivers/md/dm-thin.c

@@ -4249,6 +4249,10 @@ static void thin_io_hints(struct dm_target *ti, struct queue_limits *limits)
 {
 	struct thin_c *tc = ti->private;
 	struct pool *pool = tc->pool;
+	struct queue_limits *pool_limits = dm_get_queue_limits(pool->pool_md);
+
+	if (!pool_limits->discard_granularity)
+		return; /* pool's discard support is disabled */
 
 	limits->discard_granularity = pool->sectors_per_block << SECTOR_SHIFT;
 	limits->max_discard_sectors = 2048 * 1024 * 16; /* 16G */