xprtrdma: Clean up physical_op_map()

physical_op_unmap{_sync} don't use mr_nsegs, so don't bother to set
it in physical_op_map.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>