bcachefs: Add a kthread_should_stop() check to allocator thread

Turns out it's possible during shutdown for the allocator to get stuck
spinning on bch2_invalidate_buckets() without hitting any of the other
checks.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -1104,6 +1104,8 @@ static int bch2_allocator_thread(void *arg)
 
 	while (1) {
 		cond_resched();
+		if (kthread_should_stop())
+			break;
 
 		pr_debug("discarding %zu invalidated buckets",
 			 fifo_used(&ca->free_inc));