bcachefs: Release transaction before wake up

We will get this if we wake up first:

Kernel panic - not syncing: btree_node_write_done leaked btree_trans

since there are still transactions waiting for cycle detectors after
BTREE_NODE_write_in_flight is cleared.
Signed-off-by: Alan Huang <mmpgouride@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_io.c

@@ -1838,10 +1838,11 @@ static void btree_node_write_done(struct bch_fs *c, struct btree *b)
 	struct btree_trans *trans = bch2_trans_get(c);
 
 	btree_node_lock_nopath_nofail(trans, &b->c, SIX_LOCK_read);
-	__btree_node_write_done(c, b);
-	six_unlock_read(&b->c.lock);
 
+	/* we don't need transaction context anymore after we got the lock. */
 	bch2_trans_put(trans);
+	__btree_node_write_done(c, b);
+	six_unlock_read(&b->c.lock);
 }
 
 static void btree_node_write_work(struct work_struct *work)