lookup_export: really prefer nonV4ROOT exports.

lookup_export() claims to "Always prefer non-V4ROOT mounts" (meaning
"exports") but actually prefers V4ROOT exports - once it has 'found'
one it will never replace it.

So fix that inversion, and add code so that it proactively prefers a
non-V4ROOT whether it is found before or after a V4ROOT.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>