nfsd: stop READDIRPLUS returning inconsistent attributes

The NFSv3 READDIRPLUS gets some of the returned attributes from the
readdir, and some from an inode returned from a new lookup.  The two
objects could be different thanks to intervening renames.

The attributes in READDIRPLUS are optional, so let's just skip them if
we notice this case.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>