NFS: Allow applications to speed up readdir+statx() using AT_STATX_DONT_SYNC

If the application uses the AT_STATX_DONT_SYNC flag after doing readdir(),
then we should still mark the parent inode as seeing a readdirplus hit.
That ensures that we continue to use readdirplus in the 'ls -l' type
of workflow to do fast lookups of the dentries.

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