um: Implement TRACE_IRQFLAGS_SUPPORT

UML enables TRACE_IRQFLAGS_SUPPORT but doesn't actually implement
it. It seems to have been added for lockdep support, but that can't
actually really work well without IRQ flags tracing, as is also
very noisily reported when enabling CONFIG_DEBUG_LOCKDEP.

Implement it now.

Fixes: 711553efa5 ("[PATCH] uml: declare in Kconfig our partial LOCKDEP support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/main.c

@@ -170,7 +170,7 @@ int __init main(int argc, char **argv, char **envp)
 	 * that they won't be delivered after the exec, when
 	 * they are definitely not expected.
 	 */
-	unblock_signals();
+	unblock_signals_trace();
 
 	os_info("\n");
 	/* Reboot */