NetworkPkg/IScsiDxe: clean up "ISCSI_CHAP_AUTH_DATA.OutChallengeLength"
    
    The "ISCSI_CHAP_AUTH_DATA.OutChallenge" field is declared as a UINT8 array
    with ISCSI_CHAP_AUTH_MAX_LEN (1024) elements. However, when the challenge
    is generated and formatted, only ISCSI_CHAP_RSP_LEN (16) octets are used
    in the array.
    
    Change the array size to ISCSI_CHAP_RSP_LEN, and remove the (now unused)
    ISCSI_CHAP_AUTH_MAX_LEN macro.
    
    Remove the "ISCSI_CHAP_AUTH_DATA.OutChallengeLength" field, which is
    superfluous too.
    
    Most importantly, explain in a new comment *why* tying the challenge size
    to the digest size (ISCSI_CHAP_RSP_LEN) has always made sense. (See also
    Linux kernel commit 19f5f88ed779, "scsi: target: iscsi: tie the challenge
    length to the hash digest size", 2019-11-06.) For sure, the motivation
    that the new comment now explains has always been there, and has always
    been the same, for IScsiDxe; it's just that now we spell it out too.
    
    No change in peer-visible behavior.
    
    Cc: Jiaxin Wu <jiaxin.wu@intel.com>
    Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Cc: Siyuan Fu <siyuan.fu@intel.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3356
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
    Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
    Message-Id: <20210608121259.32451-4-lersek@redhat.com>