btrfs: Remove fs_info argument from btrfs_update_commit_device_bytes_used

We already pass the btrfs_transaction which references fs_info so no
need to pass the later as an argument. Also use the opportunity to
shorten transaction->trans. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>