kernel/selinux-testsuite: exclude userfaultfd on old kernels
    
    On RHEL-7 (and other old kernels), __NR_userfaultfd may not be defined
    on some arches, so add a kernel version check for now to exclude
    userfaultfd test when it would be skipped anyway.
    
    I'll fix this more nicely in upstream, this is just a temporary solution
    until that gets merged.
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>