xprtrdma: Add ro_unmap_sync method for all-physical registration

physical's ro_unmap is synchronous already. The new ro_unmap_sync
method just has to DMA unmap all MRs associated with the RPC
request.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Devesh Sharma <devesh.sharma@avagotech.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>