bpf: Simplify bool comparison

Fix the following coccicheck warning:

./tools/bpf/bpf_dbg.c:893:32-36: WARNING: Comparison to bool.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1612777416-34339-1-git-send-email-jiapeng.chong@linux.alibaba.com