Add nid_namelen bounds check to nfssvc system call

This is only allowed by root and only used by the nfs daemon, which
should not provide an incorrect value. However, it's still good
practice to validate data provided by userland.

PR: 206626
Reported by: CTurt <cturt@hardenedbsd.org>
Reviewed by: rmacklem
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D6201