Btrfs: simpler and more efficient cleanup of a log tree's extent io tree

We currently are in a loop finding each range (corresponding to a btree
node/leaf) in a log root's extent io tree and then clean it up. This is a
waste of time since we are traversing the extent io tree's rb_tree more
times then needed (one for a range lookup and another for cleaning it up)
without any good reason.

We free the log trees when we are in the critical section of a transaction
commit (the transaction state is set to TRANS_STATE_COMMIT_DOING), so it's
of great convenience to do everything as fast as possible in order to
reduce the time we block other tasks from starting a new transaction.

So fix this by traversing the extent io tree once and cleaning up all its
records in one go while traversing it.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>