Fix ztest deadlock in spa_vdev_remove()

Fix ztest deadlock in spa_vdev_remove()

This patch corrects an issue where spa_vdev_remove() would
call spa_history_log_internal() while holding the spa config
lock. This function may decide to block until the next txg if
the current one seems too full. However, since the thread is
holding the config log, the txg sync thread cannot progress
and the system ends up deadlocked. This patch simply moves
all calls to spa_history_log_internal() outside of the config
lock.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8162