Fix vdev removal finishing race

This patch fixes a race condition where the end of
vdev_remove_replace_with_indirect(), which holds
svr_lock, would race against spa_vdev_removal_destroy(),
which destroys the same lock and is called asynchronously
via dsl_sync_task_nowait().

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Issue #6900
Closes #8083