SUNRPC: Add "@len" parameter to gss_unwrap()

Refactor: This is a pre-requisite to fixing the client-side ralign
computation in gss_unwrap_resp_priv().

The length value is passed in explicitly rather that as the value
of buf->len. This will subsequently allow gss_unwrap_kerberos_v1()
to compute a slack and align value, instead of computing it in
gss_unwrap_resp_priv().

Fixes: 35e77d21baa0 ("SUNRPC: Add rpc_auth::au_ralign field")
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 25fbd8d..7885f37 100644
--- a/net/sunrpc/auth_gss/auth_gss.c
+++ b/net/sunrpc/auth_gss/auth_gss.c
@@ -2043,9 +2043,9 @@ gss_unwrap_resp_priv(struct rpc_task *task, struct rpc_cred *cred,
 	offset = (u8 *)(p) - (u8 *)head->iov_base;
 	if (offset + opaque_len > rcv_buf->len)
 		goto unwrap_failed;
-	rcv_buf->len = offset + opaque_len;
 
-	maj_stat = gss_unwrap(ctx->gc_gss_ctx, offset, rcv_buf);
+	maj_stat = gss_unwrap(ctx->gc_gss_ctx, offset,
+			      offset + opaque_len, rcv_buf);
 	if (maj_stat == GSS_S_CONTEXT_EXPIRED)
 		clear_bit(RPCAUTH_CRED_UPTODATE, &cred->cr_flags);
 	if (maj_stat != GSS_S_COMPLETE)