lib/error-inject: traverse list with mutex

Traversing list without mutex in get_injectable_error_type will
race with the following code:
    list_del_init(&ent->list)
    kfree(ent)
in module_unload_ei_list. So fix that.

Link: https://lkml.kernel.org/r/20220620100244.82896-1-wuchi.zero@gmail.com


Signed-off-by: wuchi <wuchi.zero@gmail.com>
Cc: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>