btrfs: fix non-empty delayed iputs list on unmount due to async workers

btrfs: fix non-empty delayed iputs list on unmount due to async workers

At close_ctree() after we have ran delayed iputs either explicitly through
calling btrfs_run_delayed_iputs() or later during the call to
btrfs_commit_super() or btrfs_error_commit_super(), we assert that the
delayed iputs list is empty.

We have (another) race where this assertion might fail because we have
queued an async write into the fs_info->workers workqueue. Here's how it
happens:

1) We are submitting a data bio for an inode that is not the data
   relocation inode, so we call btrfs_wq_submit_bio();

2) btrfs_wq_submit_bio() submits a work for the fs_info->workers queue
   that will run run_one_async_done();

3) We enter close_ctree(), flush several work queues except
   fs_info->workers, explicitly run delayed iputs with a call to
   btrfs_run_delayed_iputs() and then again shortly after by calling
   btrfs_commit_super() or btrfs_error_commit_super(), which also run
   delayed iputs;

4) run_one_async_done() is executed in the work queue, and because there
   was an IO error (bio->bi_status is not 0) it calls btrfs_bio_end_io(),
   which drops the final reference on the associated ordered extent by
   calling btrfs_put_ordered_extent() - and that adds a delayed iput for
   the inode;

5) At close_ctree() we find that after stopping the cleaner and
   transaction kthreads the delayed iputs list is not empty, failing the
   following assertion:

      ASSERT(list_empty(&fs_info->delayed_iputs));

Fix this by flushing the fs_info->workers workqueue before running delayed
iputs at close_ctree().

David reported this when running generic/648, which exercises IO error
paths by using the DM error table.

Reported-by: David Sterba <dsterba@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>