bcachefs: Fix shutdown ordering

the btree key cache uses the srcu struct created/destroyed by
btree_iter.c; btree_iter needs to be exited last.

Reported-by: syzbot+3af9daea347788b15213@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/printbuf.c

@@ -45,6 +45,13 @@ int bch2_printbuf_make_room(struct printbuf *out, unsigned extra)
 
 	unsigned new_size = roundup_pow_of_two(out->size + extra);
 
+	/* Sanity check... */
+	if (new_size > PAGE_SIZE << MAX_PAGE_ORDER) {
+		out->allocation_failure = true;
+		out->overflow = true;
+		return -ENOMEM;
+	}
+
 	/*
 	 * Note: output buffer must be freeable with kfree(), it's not required
 	 * that the user use printbuf_exit().

fs/bcachefs/super.c

@@ -551,9 +551,9 @@ static void __bch2_fs_free(struct bch_fs *c)
 	bch2_fs_io_read_exit(c);
 	bch2_fs_buckets_waiting_for_journal_exit(c);
 	bch2_fs_btree_interior_update_exit(c);
-	bch2_fs_btree_iter_exit(c);
 	bch2_fs_btree_key_cache_exit(&c->btree_key_cache);
 	bch2_fs_btree_cache_exit(c);
+	bch2_fs_btree_iter_exit(c);
 	bch2_fs_replicas_exit(c);
 	bch2_fs_journal_exit(&c->journal);
 	bch2_io_clock_exit(&c->io_clock[WRITE]);