nfsd_fh: if two exports are possible, choose the one without V4ROOT

When nfsd_fh it looking for an export for a particular
client and file-handle, it might find two exports for the same path:
one with NFSEXP_V4ROOT, one with out.

As nfsd_fh calls cache_export_ent to give the export information to
the kernel it much choose the same export that auth_authenticate
chooses for get_rootfh which it also passes cache_export_ent (via
cache_export).
i.e. it must choose the non-V4ROOT on where possible.

Also change
   strcmp(foo, bar)
to
   strcmp(foo, bar) == 0
because I have a pathological fear of the former.

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