NFSv4.2: llseek(SEEK_HOLE) and llseek(SEEK_DATA) don't require data sync

We want to ensure that we write the cached data to the server, but
don't require it be synced to disk. If the server reboots, we will
get a stateid error, which will cause us to retry anyway.

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