Removing unneeded mutex for reading vq_pending_tree size

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) the worst case would be that a single read could be queued on a mirror side which more busy than thought

The benefit of this change is streamlining of the code path since
it is taken for *every* mirror member on *every* read.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1739