bcachefs: Use trylock in bch2_prt_backtrace()

Easy workaround for a lockdep splat - and since bch2_prt_backtrace() is
only used in debug code this is fine.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/util.c

@@ -270,11 +270,9 @@ int bch2_prt_backtrace(struct printbuf *out, struct task_struct *task)
 {
 	unsigned long entries[32];
 	unsigned i, nr_entries;
-	int ret;
 
-	ret = down_read_killable(&task->signal->exec_update_lock);
-	if (ret)
-		return ret;
+	if (!down_read_trylock(&task->signal->exec_update_lock))
+		return 0;
 
 	nr_entries = stack_trace_save_tsk(task, entries, ARRAY_SIZE(entries), 0);
 	for (i = 0; i < nr_entries; i++) {