ARM: KVM: sanitize freeing of HYP page tables

Instead of trying to free everything from PAGE_OFFSET to the
top of memory, use the virt_addr_valid macro to check the
upper limit.

Also do the same for the vmalloc region where the IO mappings
are allocated.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>