fix casting constant to _Bool

Casting to _Bool requires a zero test rather than truncation.  Also add
a new cast warning for _Bool since this is not about truncation.

Cc: Pekka Enberg <penberg@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>