MdeModulePkg: Add old IPv4_DEVICE_PATH support for new IScsiDxe

MdeModulePkg: Add old IPv4_DEVICE_PATH support for new IScsiDxe

GatewayIpAddress and SubnetMask do not exist in old IPv4_DEVICE_PATH, this
will lead new IScsiDxe to error if IPv4_DEVICE_PATH in system is not
updated.

Following UEFI2.5 spec of IPv4_DEVICE_PATH do a check before accessing
fields only defined in new version, add a judgement here to make old
IPv4_DEVICE_PATH and new IScsiDxe can cowork.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: fanwang2 <fan.wang@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
[lersek@redhat.com: rewrapped commit message]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18057 6f19259b-4bc3-4df7-8a09-765794883524