KVM: selftests: Silence compiler warning in the kvm_page_table_test

When compiling kvm_page_table_test.c, I get this compiler warning
with gcc 11.2:

kvm_page_table_test.c: In function 'pre_init_before_test':
../../../../tools/include/linux/kernel.h:44:24: warning: comparison of
 distinct pointer types lacks a cast
   44 |         (void) (&_max1 == &_max2);              \
      |                        ^~
kvm_page_table_test.c:281:21: note: in expansion of macro 'max'
  281 |         alignment = max(0x100000, alignment);
      |                     ^~~

Fix it by adjusting the type of the absolute value.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20220414103031.565037-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>