nfsd: Sanity check the len argument for ListXattr

The check for the original len being >= retlen needs to
be done before the "if (nd->nd_repstat == 0)" code, so
that it can be reported as too small.

Reported by: rtm@lcs.mit.edu
Tested by: rtm@lcs.mit.edu
PR: 260046
MFC after: 2 weeks