x86 sysenter: clear %ebp on exit.

It contains the thread info pointer. That's not something that
user mode can really use for anything interesting, but it's also
not something that user mode should ever really see.

Pointed out by Brad Spender as being in PaX.

arch/i386/kernel/entry.S

@@ -235,6 +235,7 @@ sysenter_past_esp:
 /* if something modifies registers it must also disable sysexit */
 	movl EIP(%esp), %edx
 	movl OLDESP(%esp), %ecx
+	xorl %ebp,%ebp
 	sti
 	sysexit