arm64: avoid clang warning about self-assignment

Building a preprocessed source file for arm64 now always produces
a warning with clang because of the page_to_virt() macro assigning
a variable to itself.

Adding a new temporary variable avoids this issue.

Fixes: 2813b9c0

 ("kasan, mm, arm64: tag non slab memory allocated via pagealloc")
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>