btrfs: remove btrfs_fs_info::compressed_write_workers

btrfs: remove btrfs_fs_info::compressed_write_workers

The reason why end_bbio_compressed_write() queues a work into
compressed_write_workers wq is for end_compressed_writeback() call, as
it will grab all the involved folios and clear the writeback flags,
which may sleep.

However now we always run btrfs_bio::end_io() in task context, there is
no need to queue the work anymore.

Just remove btrfs_fs_info::compressed_write_workers and
compressed_bio::write_end_work.

There is a comment about the works queued into
compressed_write_workers, now change to flush endio wq instead, which is
responsible to handle all data endio functions.

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