arm64: kaslr: Reserve size of ARM64_MEMSTART_ALIGN in linear region

When KASLR is enabled (CONFIG_RANDOMIZE_BASE=y), the top 4K of kernel
virtual address space may be mapped to physical addresses despite being
reserved for ERR_PTR values.

Fix the randomization of the linear region so that we avoid mapping the
last page of the virtual address space.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: liyueyi <liyueyi@live.com>
[will: rewrote commit message; merged in suggestion from Ard]
Signed-off-by: Will Deacon <will.deacon@arm.com>