bcachefs: Fix initialization order for srcu barrier

btree_iter_init() needs to happen before key_cache_init(), to initialize
btree_trans_barrier

Reported-by: syzbot+3cca837c2183f8f6fcaf@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -912,9 +912,9 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 	    bch2_io_clock_init(&c->io_clock[WRITE]) ?:
 	    bch2_fs_journal_init(&c->journal) ?:
 	    bch2_fs_replicas_init(c) ?:
+	    bch2_fs_btree_iter_init(c) ?:
 	    bch2_fs_btree_cache_init(c) ?:
 	    bch2_fs_btree_key_cache_init(&c->btree_key_cache) ?:
-	    bch2_fs_btree_iter_init(c) ?:
 	    bch2_fs_btree_interior_update_init(c) ?:
 	    bch2_fs_buckets_waiting_for_journal_init(c) ?:
 	    bch2_fs_btree_write_buffer_init(c) ?: