livepatch/arm64: use WARN_ON() for arch_klp_unpatch_func errors

hulk inclusion
category: bugfix
bugzilla: 5507
CVE: NA

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BUG_ON will cause the kernel to crash, using WARN_ON instead.

Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>