NFS: merge _full and _partial read rpc_ops

Decouple nfs_pgio_header and nfs_read_data, and have (possibly
multiple) nfs_read_datas each take a refcount on nfs_pgio_header.

For the moment keeps nfs_read_header as a way to preallocate a single
nfs_read_data with the nfs_pgio_header.  The code doesn't need this,
and would be prettier without, but given the amount of churn I am
already introducing I didn't want to play with tuning new mempools.

This also fixes bug in pnfs_ld_handle_read_error.  In the case of
desc->pg_bsize < PAGE_CACHE_SIZE, the pages list was empty, causing
replay attempt to do nothing.

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/direct.c

@@ -319,10 +319,16 @@ static ssize_t nfs_direct_read_schedule_segment(struct nfs_direct_req *dreq,
 		bytes = min(rsize,count);
 
 		result = -ENOMEM;
-		rhdr = nfs_readhdr_alloc(nfs_page_array_len(pgbase, bytes));
+		rhdr = nfs_readhdr_alloc();
 		if (unlikely(!rhdr))
 			break;
-		data = &rhdr->rpc_data;
+		data = nfs_readdata_alloc(&rhdr->header, nfs_page_array_len(pgbase, bytes));
+		if (!data) {
+			nfs_readhdr_free(&rhdr->header);
+			break;
+		}
+		data->header = &rhdr->header;
+		atomic_inc(&data->header->refcnt);
 		pages = &data->pages;
 
 		down_read(&current->mm->mmap_sem);