NFS: Create a common initiate_pgio() function

Most of this code is the same for both the read and write paths, so
combine everything and use the rw_ops when necessary.

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