btrfs: convert extent-tree tracepoints to use fs_info

The extent-tree tracepoints all operate on the extent root, regardless of
which root is passed in.  Let's just use the extent root objectid instead.
If it turns out that nobody is depending on the format of this tracepoint,
we can drop the root printing entirely.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>