NFS: Ensure the server has an up to date ctime before hardlinking

Creating a hard link is required by POSIX to update the file ctime, so
ensure that the file data is synced to disk so that we don't clobber the
updated ctime by writing back after creating the hard link.

Fixes: 9f768272

 ("NFS: Move the delegation return down into nfs4_proc_link()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>