ksmbd: add low bound validation to FSCTL_SET_ZERO_DATA

Smatch static checker warning:
 fs/ksmbd/smb2pdu.c:7759 smb2_ioctl()
 warn: no lower bound on 'off'

Fix unexpected result that could caused from negative off and bfz.

Fixes: b5e5f9df ("ksmbd: check invalid FileOffset and BeyondFinalZero in FSCTL_ZERO_DATA")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/ksmbd/smb2pdu.c

@@ -7755,7 +7755,7 @@ int smb2_ioctl(struct ksmbd_work *work)
 
 		off = le64_to_cpu(zero_data->FileOffset);
 		bfz = le64_to_cpu(zero_data->BeyondFinalZero);
-		if (off > bfz) {
+		if (off < 0 || bfz < 0 || off > bfz) {
 			ret = -EINVAL;
 			goto out;
 		}