Lock xprt_list in during client op-version support check

Change-Id: Ibfa8a583048d6765743a748427447a7dbbcb3775
Perform the client-op-version check in volume set stage at the same place in
both origin and non-origin glusterd.

Change-Id: I0ba191a1b6232f6bc89bf1fecac4563f03c3327a