MdeModulePkg/SMM: Support to unregister SMI handler inside SMI handler
    
    To support unregister SMI handler inside SMI handler itself,
    get next node before SMI handler is executed, since LIST_ENTRY that
    Link points to may be freed if unregister SMI handler in SMI handler
    itself.
    
    Cc: Liming Gao <gaoliming@byosoft.com.cn>
    Cc: Jiaxin Wu <jiaxin.wu@intel.com>
    Reviewed-by: Ray Ni <ray.ni@intel.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>
    Message-Id: <20240301030133.628-2-zhiguang.liu@intel.com>