MdeModulePkg/RamDiskDxe: Restrict on RAM disk size (CVE-2018-12180)

MdeModulePkg/RamDiskDxe: Restrict on RAM disk size (CVE-2018-12180)

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1134

Originally, the block size of created Ram disks is hard-coded to 512
bytes. However, if the total size of the Ram disk is not a multiple of 512
bytes, there will be potential memory access issues when dealing with the
last block of the Ram disk.

This commit will adjust the block size of the Ram disks to ensure that the
total size is a multiple of the block size.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>