SUNRPC: Fix GSS privacy computation of auth->au_ralign

When the au_ralign field was added to gss_unwrap_resp_priv, the
wrong calculation was used. Setting au_rslack == au_ralign is
probably correct for kerberos_v1 privacy, but kerberos_v2 privacy
adds additional GSS data after the clear text RPC message.
au_ralign needs to be smaller than au_rslack in that fairly common
case.

When xdr_buf_trim() is restored to gss_unwrap_kerberos_v2(), it does
exactly what I feared it would: it trims off part of the clear text
RPC message. However, that's because rpc_prepare_reply_pages() does
not set up the rq_rcv_buf's tail correctly because au_ralign is too
large.

Fixing the au_ralign computation also corrects the alignment of
rq_rcv_buf->pages so that the client does not have to shift reply
data payloads after they are received.

Fixes: 35e77d21ba ("SUNRPC: Add rpc_auth::au_ralign field")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/gss_api.h

@@ -21,6 +21,7 @@
 struct gss_ctx {
 	struct gss_api_mech	*mech_type;
 	void			*internal_ctx_id;
+	unsigned int		slack, align;
 };
 
 #define GSS_C_NO_BUFFER		((struct xdr_netobj) 0)