NFS: Create a common rw_header_alloc and rw_header_free function

I create a new struct nfs_rw_ops to decide the differences between reads
and writes.  This struct will be set when initializing a new
nfs_pgio_descriptor, and then passed on to the nfs_rw_header when a new
header is allocated.

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

include/linux/nfs_xdr.h

@@ -1269,6 +1269,7 @@ struct nfs_pgio_header {
 	const struct rpc_call_ops *mds_ops;
 	void (*release) (struct nfs_pgio_header *hdr);
 	const struct nfs_pgio_completion_ops *completion_ops;
+	const struct nfs_rw_ops	*rw_ops;
 	struct nfs_direct_req	*dreq;
 	void			*layout_private;
 	spinlock_t		lock;