um: Always dump trace for specified task in show_stack

Currently, show_stack() always dumps the trace of the current task.
However, it should dump the trace of the specified task if one is
provided. Otherwise, things like running "echo t > sysrq-trigger"
won't work as expected.

Fixes: 970e51feaddb ("um: Add support for CONFIG_STACKTRACE")
Signed-off-by: Tiwei Bie <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/kernel/sysrq.c

···
 	}
 
 	printk("%sCall Trace:\n", loglvl);
-	dump_trace(current, &stackops, (void *)loglvl);
+	dump_trace(task ?: current, &stackops, (void *)loglvl);
 }