KVM: arm64: selftests: Fix incorrect rounding in page_align()

The implementation of `page_align()` in `processor.c` calculates
alignment incorrectly for values that are already aligned. Specifically,
`(v + vm->page_size) & ~(vm->page_size - 1)` aligns to the *next* page
boundary even if `v` is already page-aligned, potentially wasting a page
of memory.

Fix the calculation to use standard alignment logic: `(v + vm->page_size
- 1) & ~(vm->page_size - 1)`.

Fixes: 7a6629e ("kvm: selftests: add virt mem support for aarch64")
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260109082218.3236580-3-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

tools/testing/selftests/kvm/lib/arm64/processor.c

@@ -23,7 +23,7 @@ static vm_vaddr_t exception_handlers;
 
 static uint64_t page_align(struct kvm_vm *vm, uint64_t v)
 {
-	return (v + vm->page_size) & ~(vm->page_size - 1);
+	return (v + vm->page_size - 1) & ~(vm->page_size - 1);
 }
 
 static uint64_t pgd_index(struct kvm_vm *vm, vm_vaddr_t gva)