kasan, arm64: move initialization message

Tag-based KASAN modes are fully initialized with kasan_init_tags(),
while the generic mode only requireds kasan_init(). Move the
initialization message for tag-based modes into kasan_init_tags().

Also fix pr_fmt() usage for KASAN code: generic mode doesn't need it,
tag-based modes should use "kasan:" instead of KBUILD_MODNAME.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://linux-review.googlesource.com/id/Idfd1e50625ffdf42dfc3dbf7455b11bd200a0a49