[PATCH] NFS: Ensure that we revalidate the cached file length for llseek(SEEK_END)

This fixes a data corruption error for mail delivery applications that
expect to be able to do posix locking and then append writes on NFS.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>