bcachefs: Fix journal_buf_realloc()

It used to be safe to reallocate a buf that the write path owns without
holding the journal lock, but now this can trigger an assertion in
journal_seq_to_buf().
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/journal_io.c

@@ -1051,9 +1051,13 @@ static void journal_buf_realloc(struct journal *j, struct journal_buf *buf)
 		return;
 
 	memcpy(new_buf, buf->data, buf->buf_size);
-	kvpfree(buf->data, buf->buf_size);
-	buf->data	= new_buf;
-	buf->buf_size	= new_size;
+
+	spin_lock(&j->lock);
+	swap(buf->data,		new_buf);
+	swap(buf->buf_size,	new_size);
+	spin_unlock(&j->lock);
+
+	kvpfree(new_buf, new_size);
 }
 
 static inline struct journal_buf *journal_last_unwritten_buf(struct journal *j)