cifsd: fix memdup.cocci warnings

mainline inclusion
from mainline-5.15-rc1
commit 86f52978
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I60T7G
CVE: NA

Reference: https://git.kernel.org/torvalds/linux/c/86f52978465b

-------------------------------

fs/cifsd/smb2pdu.c:1177:27-34: WARNING opportunity for kmemdup

 Use kmemdup rather than duplicating its implementation

Generated by: scripts/coccinelle/api/memdup.cocci
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NNamjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>
Signed-off-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NZhong Jinghua <zhongjinghua@huawei.com>

fs/cifsd/smb2pdu.c

@@ -1161,13 +1161,11 @@ static int alloc_preauth_hash(struct ksmbd_session *sess,
 	if (sess->Preauth_HashValue)
 		return 0;
 
-	sess->Preauth_HashValue = kmalloc(PREAUTH_HASHVALUE_SIZE, GFP_KERNEL);
+	sess->Preauth_HashValue = kmemdup(conn->preauth_info->Preauth_HashValue,
+			PREAUTH_HASHVALUE_SIZE, GFP_KERNEL);
 	if (!sess->Preauth_HashValue)
 		return -ENOMEM;
 
-	memcpy(sess->Preauth_HashValue,
-	       conn->preauth_info->Preauth_HashValue,
-	       PREAUTH_HASHVALUE_SIZE);
 	return 0;
 }