bcachefs: Add missing synchronize_srcu_expedited() call when shutting down

We use the polling interface to srcu for tracking pending frees; when
shutting down we don't need to wait for an srcu barrier to free them,
but SRCU still gets confused if we shutdown with an outstanding grace
period.

Reported-by: syzbot+6a038377f0a594d7d44e@syzkaller.appspotmail.com
Reported-by: syzbot+0ece6edfd05ed20e32d9@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Author: Kent Overstreet

fs/bcachefs/btree_iter.c

@@ -3401,8 +3401,10 @@ void bch2_fs_btree_iter_exit(struct bch_fs *c)
 		bch2_time_stats_exit(&s->lock_hold_times);
 	}
 
-	if (c->btree_trans_barrier_initialized)
+	if (c->btree_trans_barrier_initialized) {
+		synchronize_srcu_expedited(&c->btree_trans_barrier);
 		cleanup_srcu_struct(&c->btree_trans_barrier);
+	}
 	mempool_exit(&c->btree_trans_mem_pool);
 	mempool_exit(&c->btree_trans_pool);
 }