Removing unneeded mutex for reading vq->vq_pending_tree->avl_numnodes

locking mutex &vq->vq_lock in vdev_mirror_pending is unneeded:
* no data is modified
* only vq_pending_tree is read
* in case garbage is returned (eg. vq_pending_tree being updated while the read is made) worst case would be that a single read could be queued on a mirror side which more busy than thought

Benefit of this change is streamlining of the codepath, since it is taken for *every* mirror member on *every* read.