[CVE-2011-4110] fix panic replacing user keys

[Andy Whitcroft]

CVE-2011-4110
	There is a NULL pointer deref in the user-defined key type whereby
	updating a negative key into a fully instantiated key will cause
	an oops to occur when the code attempts to free the non-existent
	old payload.

Fixes for this have hit oneiric and later via mainline and stable.
Following this email is a patch for hardy, lucid, lucid/fsl-imx51,
maverick, maverick/ti-omap4, natty, and natty/ti-omap4.  This is a simple
backport from the mainline commit.

Proposing for hardy, lucid, lucid/fsl-imx51, maverick, maverick/ti-omap4,
natty, and natty/ti-omap4.

-apw