NFS: Be more aggressive in using readdirplus for 'ls -l' situations

Try to detect 'ls -l' by having nfs_getattr() look at whether or not
there is an opendir() file descriptor for the parent directory.
If so, then assume that we want to force use of readdirplus in order
to avoid the multiple GETATTR calls over the wire.

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