static_key_enabled() can be used on struct static_key but not on its
wrapper types static_key_true and static_key_false.  The function is
useful for debugging and management of static keys.  Update it so that
it can be used for the wrapper types too.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>