Btrfs: check return value of commit when recovering log

We need to check the return value of the commit in case something goes wrong,
otherwise we could end up going down the line and doing more stuff (like orphan
cleanup) before we notice we should have errored out.  We need to do this before
we free up the log_tree_root since the caller will handle all of that.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>