lguest: clear cached last cpu when guest_set_pgd() called.

commit v3.9-rc1-53-g6d0cda9 "lguest: cache last cpu we ran on." missed
one case, which causes a triple fault.  The guest calls guest_set_pgd()
on the top page, and we carefully remap the Switcher text page.  But
we didn't reset last_host_cpu, so map_switcher_in_guest() thinks
the guest's regs and IDT/GDT etc are already mapped.

Reported-by: Paul Bolle <pebolle@tiscali.nl>
Tested-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/page_tables.c

@@ -1002,6 +1002,7 @@ void guest_set_pgd(struct lguest *lg, unsigned long gpgdir, u32 idx)
 			kill_guest(&lg->cpus[0],
 				   "Cannot populate switcher mapping");
 		}
+		lg->pgdirs[pgdir].last_host_cpu = -1;
 	}
 }