try to fix lor between z_teardown_lock and spa_namespace_lock

The lock order reversal and a resulting deadlock were introduced
in r285021 / D2865. The problem is that zfs_register_callbacks() calls
dsl_prop_get_integer() that has to acquire spa_namespace_lock.
At the same time, spa_config_sync() is called with spa_namespace_lock
held and then it performs ZFS vnode operations that acquire
z_teardown_lock in the reader mode.

So, fix the problem by using dsl_prop_get_int_ds() instead of
dsl_prop_get_integer(). The former does not need to look up
the pool and the dataset by name.

Reported by: many
Reviewed by: delphij
Tested by: delphij, Jens Schweikhardt <schweikh@schweikhardt.net>
MFC after: 5 days
X-MFC with: r285021