NFS: Optimise away unnecessary setattrs for open(O_TRUNC);

Currently, we will correctly optimise away a truncate that doesn't
change the file size. However, in the case of open(O_TRUNC), we
also want to optimise away the time changes.

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

diff --cc fs/nfs/dir.c
Simple merge

diff --cc fs/nfs/inode.c
Simple merge

diff --cc fs/nfs/nfs4proc.c
Simple merge