Note: When upper layer's read/write request cannot be fulfilled, the block
layout driver shouldn't silently mark the page as error. It should do
what can be done and  leave the rest to the upper layer. To do so, we
should set rdata/wdata->res.count properly.

When upper layer re-send the read/write request to finish the rest
part of the request, pgbase is the position where we should start at.

[pnfsblock: mark IO error with NFS_LAYOUT_{RW|RO}_FAILED]
Signed-off-by: NPeng Tao <peng_tao@emc.com>
[pnfsblock: read path error handling]
Signed-off-by: NFred Isaman <iisaman@citi.umich.edu>
[pnfsblock: handle errors when read or write pagelist.]
Signed-off-by: NZhang Jingwang <yyalone@gmail.com>
[pnfs-block: use new read_pagelist api]
Signed-off-by: NBenny Halevy <bhalevy@panasas.com>
Signed-off-by: NBenny Halevy <bhalevy@tonian.com>
Signed-off-by: NJim Rees <rees@umich.edu>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>