btrfs: remove pointless 'ref_root' variable from run_delayed_data_ref()

The 'ref_root' variable, at run_delayed_data_ref(), is not really needed
as we can always use ref->root directly, plus its initialization to 0 is
completely pointless as we assign it ref->root before its first use.
So just drop that variable and use ref->root directly.

This may help avoid some warnings with clang tools such as the one
reported/fixed by commit 966de47f ("btrfs: remove redundant
initialization of variables in log_new_ancestors").

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