Historically the fast-path static key `kasan_flag_enabled` existed
only for `CONFIG_KASAN_HW_TAGS`. Generic and SW_TAGS either relied on
`kasan_arch_is_ready()` or evaluated KASAN checks unconditionally.
As a result every architecture had to toggle a private flag
in its `kasan_init()`.
This patch turns the flag into a single global runtime predicate that
is built for every `CONFIG_KASAN` mode and adds a helper that flips
the key once KASAN is ready.