btrfs: use verbose assert at peek_discard_list()

We now have a verbose variant of ASSERT() so that we can print the value
of the block group's discard_index. So use it for better problem analysis
in case the assertion is triggered.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/discard.c

@@ -256,7 +256,9 @@ static struct btrfs_block_group *peek_discard_list(
 				 * the discard lists.
 				 */
 				ASSERT(block_group->discard_index !=
-				       BTRFS_DISCARD_INDEX_UNUSED);
+				       BTRFS_DISCARD_INDEX_UNUSED,
+				       "discard_index=%d",
+				       block_group->discard_index);
 			} else {
 				list_del_init(&block_group->discard_list);
 				btrfs_put_block_group(block_group);