kasan: respect CONFIG_KASAN_VMALLOC for kasan_flag_vmalloc

kasan: respect CONFIG_KASAN_VMALLOC for kasan_flag_vmalloc

Never enable the kasan_flag_vmalloc static branch unless
CONFIG_KASAN_VMALLOC is enabled.

This does not fix any observable bugs (vmalloc annotations for the HW_TAGS
mode are no-op with CONFIG_KASAN_VMALLOC disabled) but rather just cleans
up the code.

Link: https://lkml.kernel.org/r/3e5c933c8f6b59bd587efb05c407964be951772c.1703188911.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>