btrfs: don't double-defer bio completions for compressed reads

The bio completion handler of the bio used for the compressed data is
already run in a workqueue using btrfs_bio_wq_end_io, so don't schedule
the completion of the original bio to the same workqueue again but just
execute it directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>