KCM: Do not use 2048 as fixed size for the payload
    
    The KCM code has the limit set as 2048 only inside #ifdef __APPLE__,
    while it should be normally set as 10 * 1024 * 1024, as seen in:
    https://github.com/krb5/krb5/blob/master/src/lib/krb5/ccache/cc_kcm.c#L53
    
    Last but not least, doesn't make much sense to use a fixed value as the
    first 4 bytes received are the payload size ... so let's just allocate
    the needed size instead of having a fixed value.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3671
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>