nmount(2), NFS: Accept 'ngroups_max + 1' groups in "export" credentials

There is no technical reason to limit that to NGROUPS_MAX, which is off
by one and just our minimum maximum value.

Reviewed by: rmacklem
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47013