MAC: 'kernel_mac_support' module: Make an outdated comment more generic
    
    No functional change.
    
    Reviewed by:    jamie
    Approved by:    markj (mentor)
    MFC after:      5 days
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D46898
    
    (cherry picked from commit 90678c892d7b3a90339b7fc19fde16c64fe3cb70)