Fix taskq creation failure in vdev_open_children()

When creating and destroying pools in tight loop it's possible to
exhaust the number of allowed threads on a system. This results
in taskq_create() failling and a NULL dereference.

Resolve the issue by falling back to opening the vdevs all
synchronously.

Reviewed-by: Denys Rtveliashvili <denys@rtveliashvili.name>
Reviewed-by: Håkan Johansson <f96hajo@chalmers.se>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/spl#521
Closes #4637