eth: fbnic: fix memory corruption in fbnic_tlv_attr_get_string()

This code is trying to ensure that the last byte of the buffer is a NUL
terminator.  However, the problem is that attr->value[] is an array of
__le32, not char, so it zeroes out 4 bytes way beyond the end of the
buffer.  Cast the buffer to char to address this.

Fixes: e5cf510 ("eth: fbnic: Update fbnic_tlv_attr_get_string() to work like nla_strscpy()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Lee Trager <lee@trager.us>
Link: https://patch.msgid.link/2791d4be-ade4-4e50-9b12-33307d8410f6@stanley.mountain
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/meta/fbnic/fbnic_tlv.c

@@ -261,7 +261,7 @@ ssize_t fbnic_tlv_attr_get_string(struct fbnic_tlv_msg *attr, char *dst,
 		return -E2BIG;
 
 	srclen = le16_to_cpu(attr->hdr.len) - sizeof(*attr);
-	if (srclen > 0 && attr->value[srclen - 1] == '\0')
+	if (srclen > 0 && ((char *)attr->value)[srclen - 1] == '\0')
 		srclen--;
 
 	if (srclen >= dstsize) {