bpf: Use kmemdup() to replace kmalloc + memcpy

Eliminate the follow coccicheck warning:

./kernel/bpf/btf.c:6537:13-20: WARNING opportunity for kmemdup.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1639030882-92383-1-git-send-email-jiapeng.chong@linux.alibaba.com

kernel/bpf/btf.c

@@ -6534,12 +6534,11 @@ static struct bpf_cand_cache *populate_cand_cache(struct bpf_cand_cache *cands,
 		bpf_free_cands_from_cache(*cc);
 		*cc = NULL;
 	}
-	new_cands = kmalloc(sizeof_cands(cands->cnt), GFP_KERNEL);
+	new_cands = kmemdup(cands, sizeof_cands(cands->cnt), GFP_KERNEL);
 	if (!new_cands) {
 		bpf_free_cands(cands);
 		return ERR_PTR(-ENOMEM);
 	}
-	memcpy(new_cands, cands, sizeof_cands(cands->cnt));
 	/* strdup the name, since it will stay in cache.
 	 * the cands->name points to strings in prog's BTF and the prog can be unloaded.
 	 */