btrfs: Remove root argument of cleanup_transaction

The only thing the passed root is used for is:
1. get a reference to the fs_info and to
2. call trace_btrfs_transaction_commit.

We can achieve 1) by simply referring to the fs_info from passed trans
object. As far as 2) is concerned cleanup_transaction is called from
only one place and the 'root' argument passed is the one from the trans
handle. No functional changes.

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