NFS: Make stat() return updated mtimes after a write()

 The SuS states that a call to write() will cause mtime to be updated on
 the file. In order to satisfy that requirement, we need to flush out
 any cached writes in nfs_getattr().
 Speed things up slightly by not committing the writes.

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