NFS: Fix up "rm -rf"...

When a file is being scheduled for deletion by means of the sillyrename
mechanism, it makes sense to start out writeback of the dirty data as
soon as possible in order to ensure that the delete can occur. Examples of
cases where this is an issue include "rm -rf", which will busy-wait until
the file is closed, and the sillyrename completes.

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