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.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: emaste, markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53368