memdisk: Use real_addr_t for SEG:OFF fields

memdisk: Use real_addr_t for SEG:OFF fields

We used uint32_t in a few places where it's also handy
to refer to those fields' segment and offset components.
Now we can do so via the real_addr_t union type.

Additionally, there are a couple of places where we now
have pointer fields instead of uint32_t fields, because
we know what kind of struct the field is pointing to.

Signed-off-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>