NFS41: do not update isize if inode needs layoutcommit

nfs_update_inode will update isize if there is no queued pages. For pNFS,
layoutcommit is supposed to change file size on server, the same effect as queued
pages. nfs_update_inode may be called when dirty pages are written back (nfsi->npages==0)
but layoutcommit is not sent, and it will change client file size according to server
file size. Then client ends up losing what it just writes back in pNFS path.
So we should skip updating client file size if file needs layoutcommit.

Signed-off-by: Peng Tao <peng_tao@emc.com>
Cc: stable@kernel.org   [2.6.39]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>