exportfs: be careful to only return expected errors.

When nfsd calls fh_to_dentry, it expect ESTALE or ENOMEM as errors.
In particular it can be tempting to return ENOENT, but this is not
handled well by nfsd.

Rather than requiring strict adherence to error code code filesystems,
treat all unexpected error codes the same as ESTALE.  This is safest.

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