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 <[email protected]>
Reviewed-by: Håkan Johansson <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes zfsonlinux/spl#521
Closes #4637 

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