NFSv4: Don't call OPEN if we already have an open stateid for a file

If we already have a stateid with the correct open mode for a given file,
then we can reuse that stateid instead of re-issuing an OPEN call without
violating the close-to-open caching semantics.

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