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
    
    (cherry picked from commit 84337218786b13f0ebfa834d28de41dce02bc937)