pnfsblock: fix partial page buffer wirte

pnfsblock: fix partial page buffer wirte

commit fe6e1e8d9fad86873eb74a26e80a8f91f9e870b5 upstream.

If applications use flock to protect its write range, generic NFS
will not do read-modify-write cycle at page cache level. Therefore
LD should know how to handle non-sector aligned writes. Otherwise
there will be data corruption.

Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
[bwh: Backported to Linux 3.2:
 - Adjust context
 - s/wdata->pages\.npages/wdata->npages/
 - s/header->pnfs_error/wdata->pnfs_error/
 - Drop change in missing out_mds exit path]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>