KVM: arm64: Fix hyp_pool max order

The EL2 page allocator in protected mode maintains a per-pool max order
value to optimize allocations when the memory region it covers is small.
However, the max order value is currently under-estimated whenever the
number of pages in the region is a power of two. Fix the estimation.

Signed-of-by: Quentin Perret <qperret@google.com>