nfs_clrpcops.c: Check for too large a write reply

The "rlen" reply length for a Write operation/RPC
could cause trouble if a broken server replies with
too large a value.

Improve the sanity check for "rlen" to avoid this.

(cherry picked from commit 2c82cdd2e29f8ba00d4289f36f8baa1598a1ad9b)