UPSTREAM: arm64: avoid clang warning about self-assignment

(Upstream commit 366e37e4da23f9df498cc9577cadcb354f7bd431).

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: 2813b9c02962 ("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>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Bug: 135692346
Change-Id: I5efe330c68219a32286e3afb6c69019650d51631