NFS4.1: make pnfs_ld_[read|write]_done consistent

The two functions had diverged quite a bit, with the write function
being a bit more robust than the read.

However, these still break badly in the desc->pg_bsize < PAGE_CACHE_SIZE case,
as then there is nothing hanging on the data->pages list, and the resend
ends up doing nothing.  This will be fixed in a patch later in the series.

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