The nfs_rename() NFS client method flushes the buffer
cache for the from (and optionally to) files, so that any
writes are done before the rename.
This patch adds flushing of any dirty pages caused by
mmap'd writing.
To be honest, for most NFS servers, this is not necessary,
since most NFS servers do not change the file handle because
of a rename. (Whether or not this is the case can be detected
for NFSv4.1/4.2 and the client should be patched to check this
someday.)