gss_krb5: Introduce encryption type framework

Make the client and server code consistent regarding the extra buffer
space made available for the auth code when wrapping data.

Add some comments/documentation about the available buffer space
in the xdr_buf head and tail when gss_wrap is called.

Add a compile-time check to make sure we are not exceeding the available
buffer space.

Add a central function to shift head data.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/net/sunrpc/auth_gss/gss_krb5_wrap.c b/net/sunrpc/auth_gss/gss_krb5_wrap.c
index a6e9056..496281f 100644
--- a/net/sunrpc/auth_gss/gss_krb5_wrap.c
+++ b/net/sunrpc/auth_gss/gss_krb5_wrap.c
@@ -155,11 +155,9 @@
 
 	ptr = buf->head[0].iov_base + offset;
 	/* shift data to make room for header. */
+	xdr_extend_head(buf, offset, headlen);
+
 	/* XXX Would be cleverer to encrypt while copying. */
-	/* XXX bounds checking, slack, etc. */
-	memmove(ptr + headlen, ptr, buf->head[0].iov_len - offset);
-	buf->head[0].iov_len += headlen;
-	buf->len += headlen;
 	BUG_ON((buf->len - offset - headlen) % blocksize);
 
 	g_make_token_header(&kctx->mech_used,