Btrfs: hold the commit_root_sem when getting the commit root during send

We currently rely too heavily on roots being read-only to save us from just
accessing root->commit_root.  We can easily balance blocks out from underneath a
read only root, so to save us from getting screwed make sure we only access
root->commit_root under the commit root sem.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>