NFS: Cache aggressively when file is open for writing

Unless the user is using file locking, we must assume close-to-open
cache consistency when the file is open for writing. Adjust the
caching algorithm so that it does not clear the cache on out-of-order
writes and/or attribute revalidations.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>