Fix LOR between dp_config_rwlock and spa_props_lock

Fix LOR between dp_config_rwlock and spa_props_lock

Our QE team during automated API testing hit deadlock in ZFS, caused
by lock order reversal.  From one side dsl_sync_task_sync() locks
dp_config_rwlock as writer and calls spa_sync_props(), which waits
for spa_props_lock.  From another spa_prop_get() locks spa_props_lock
and then calls dsl_pool_config_enter(), trying to lock dp_config_rwlock
as reader.

This patch makes spa_prop_get() lock dp_config_rwlock before
spa_props_lock, making the order consistent.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #10553