crush: use bucket_exists() for item sanity checks

crush: use bucket_exists() for item sanity checks

This is because get_bucket() never returns 0 on error,
it always returns a non-zero value. So if you
assert(get_bucket(item)), you always get a postive answer.

Replace existence check for a specific item

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>