[PATCH] NFS: Fix the file size revalidation

 Instead of looking at whether or not the file is open for writes before
 we accept to update the length using the server value, we should rather
 be looking at whether or not we are currently caching any writes.

 Failure to do so means in particular that we're not updating the file
 length correctly after obtaining a POSIX or BSD lock.

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