brd: add cond_resched to brd_free_pages

The loop that frees all the pages can take unbounded amount of time, so
add cond_resched() to it.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/brd.c

@@ -152,6 +152,12 @@ static void brd_free_pages(struct brd_device *brd)
 
 		pos++;
 
+		/*
+		 * It takes 3.4 seconds to remove 80GiB ramdisk.
+		 * So, we need cond_resched to avoid stalling the CPU.
+		 */
+		cond_resched();
+
 		/*
 		 * This assumes radix_tree_gang_lookup always returns as
 		 * many pages as possible. If the radix-tree code changes,