Fix lock inversion in txg_sync_thread()

Fix lock inversion in txg_sync_thread()

This patch fixes a lock inversion issue in txg_sync_thread() where
the code would attempt hold the spa config lock as a reader while
holding tx->tx_sync_lock. This races with spa_vdev_remove() which
attempts to hold the tx->tx_sync_lock to assign a new tx (via
spa_history_log_internal()) while holding the spa config lock as a
writer.

Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Co-authored-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8010