NFSv4: Truncating file opens should also sync O_DIRECT writes

We don't just want to sync out buffered writes, but also O_DIRECT ones.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

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

diff --cc fs/nfs/nfs4file.c
Simple merge

diff --cc include/linux/nfs_fs.h
Simple merge