btrfs: test balance and send running in parallel

btrfs: test balance and send running in parallel

Test that send and balance can run in parallel, without failures and
producing correct results.

Before kernel 5.3 it was possible to run both operations in parallel,
however it was buggy and caused sporadic failures due to races, so it was
disabled in kernel 5.3 by commit 9e967495e0e0ae ("Btrfs: prevent send
failures and crashes due to concurrent relocation"). There is a now a
patch that enables both operations to safely run in parallel, and it has
the following subject:

    "btrfs: make send work with concurrent block group relocation"

This also serves the purpose of testing a succession of incremental send
operations, where we have a bunch of snapshots of the same subvolume and
we keep doing an incremental send using the previous snapshot as the
parent.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>