lrdp-v2: Fixe vmemmap offset when kaslr is enabled for arm64

KASLR randomizes the address of the linear kernel mapping, which
causes the starting address of vmemmap to be similarly randomized.
Use the value of memstart_addr in calculations instead of phys_offset.

Change-Id: I905c0f50e011f85871ca2406b9a5d78afe4f0dc6