btrfs: do not consider send context as valid when trying to flush qgroups

At qgroup.c:try_flush_qgroup() we are asserting that current->journal_info
is either NULL or has the value BTRFS_SEND_TRANS_STUB.

However allowing for BTRFS_SEND_TRANS_STUB makes no sense because:

1) It is misleading, because send operations are read-only and do not
   ever need to reserve qgroup space;

2) We already assert that current->journal_info != BTRFS_SEND_TRANS_STUB
   at transaction.c:start_transaction();

3) On a kernel without CONFIG_BTRFS_ASSERT=y set, it would result in
   a crash if try_flush_qgroup() is ever called in a send context, because
   at transaction.c:start_transaction we cast current->journal_info into
   a struct btrfs_trans_handle pointer and then dereference it.

So just do allow a send context at try_flush_qgroup() and update the
comment about it.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/qgroup.c

@@ -3545,11 +3545,15 @@ static int try_flush_qgroup(struct btrfs_root *root)
 	struct btrfs_trans_handle *trans;
 	int ret;
 
-	/* Can't hold an open transaction or we run the risk of deadlocking */
-	ASSERT(current->journal_info == NULL ||
-	       current->journal_info == BTRFS_SEND_TRANS_STUB);
-	if (WARN_ON(current->journal_info &&
-		    current->journal_info != BTRFS_SEND_TRANS_STUB))
+	/*
+	 * Can't hold an open transaction or we run the risk of deadlocking,
+	 * and can't either be under the context of a send operation (where
+	 * current->journal_info is set to BTRFS_SEND_TRANS_STUB), as that
+	 * would result in a crash when starting a transaction and does not
+	 * make sense either (send is a read-only operation).
+	 */
+	ASSERT(current->journal_info == NULL);
+	if (WARN_ON(current->journal_info))
 		return 0;
 
 	/*