The simplification of mte_get_random_tag() caused the introduction of the
warning below:

In file included from arch/arm64/include/asm/kasan.h:9,
                 from include/linux/kasan.h:16,
                 from mm/kasan/common.c:14:
mm/kasan/common.c: In function ‘mte_get_random_tag’:
arch/arm64/include/asm/mte-kasan.h:45:9: warning: ‘addr’ is used
                                         uninitialized [-Wuninitialized]
   45 |         asm(__MTE_PREAMBLE "irg %0, %0"
      |

Fix the warning using "=r" for the address in the asm inline.

Fixes: c8f8de4c

 ("arm64: kasan: simplify and inline MTE functions")
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>