NFS: Create a common pgio_rpc_prepare function

The read and write paths do exactly the same thing for the rpc_prepare
rpc_op.  This patch combines them together into a single function.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>