SUNRPC: Add xdr_pad_size() helper

Introduce a helper function to compute the XDR pad size of a
variable-length XDR object.

Clean up: Replace open-coded calculation of XDR pad sizes.
I'm sure I haven't found every instance of this calculation.

Signed-off-by: Chuck Lever <[email protected]>
diff --git a/net/sunrpc/auth_gss/auth_gss.c b/net/sunrpc/auth_gss/auth_gss.c
index 24ca861..ee060d5 100644
--- a/net/sunrpc/auth_gss/auth_gss.c
+++ b/net/sunrpc/auth_gss/auth_gss.c
@@ -1877,7 +1877,7 @@ static int gss_wrap_req_priv(struct rpc_cred *cred, struct gss_cl_ctx *ctx,
 	else
 		iov = snd_buf->head;
 	p = iov->iov_base + iov->iov_len;
-	pad = 3 - ((snd_buf->len - offset - 1) & 3);
+	pad = xdr_pad_size(snd_buf->len - offset);
 	memset(p, 0, pad);
 	iov->iov_len += pad;
 	snd_buf->len += pad;