Fix self-healing IO prior to dsl_pool_init() completion

Fix self-healing IO prior to dsl_pool_init() completion

Async writes triggered by a self-healing IO may be issued before the
pool finishes the process of initialization.  This results in a NULL
dereference of `spa->spa_dsl_pool` in vdev_queue_max_async_writes().

George Wilson recommended addressing this issue by initializing the
passed `dsl_pool_t **` prior to dmu_objset_open_impl().  Since the
caller is passing the `spa->spa_dsl_pool` this has the effect of
ensuring it's initialized.

However, since this depends on the caller knowing they must pass
the `spa->spa_dsl_pool` an additional NULL check was added to
vdev_queue_max_async_writes().  This guards against any future
restructuring of the code which might result in dsl_pool_init()
being called differently.

Signed-off-by: GeLiXin <47034221@qq.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4652