test-bdrv-drain: Fix data races

test-bdrv-drain: Fix data races

This patch addresses potential data races involving access to Job fields
in the test-bdrv-drain test.

Fixes: 7253220de4 ("test-bdrv-drain: Test drain vs. block jobs")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2900
Signed-off-by: Vitalii Mordan <mordan@ispras.ru>
Message-ID: <20250402102119.3345626-1-mordan@ispras.ru>
[kwolf: Fixed up coding style and one missing atomic access]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>