kdump-lib.sh: fix the case if no enough total RAM for kdump in get_recommend_size()
    
    For crashkernel=auto policy, if total RAM size is under a throttle,
    there is no memory reserved for kdump.
    
    Also correct a trivial bug by correcting the arch name.
    
    Signed-off-by: Pingfan Liu <piliu@redhat.com>
    Acked-by: Kairui Song <kasong@redhat.com>