kernel/synflood: refactor main test condition
    
    It turns out there is too much variability in the percent values across
    arches/machines, so give up on the general approach (checking that all
    SELinux functions are below some hard limit) and instead focus only on
    selinux_peerlbl_enabled() and compare its position with other functions.
    
    In case selinux_peerlbl_enabled() is not found in /proc/kallsyms, it is
    inlined into other functions, so we need to check selinux_ip_postroute()
    instead of selinux_peerlbl_enabled().
    
    Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>