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

tests/zfs-tests/include/commands.cfg.in

@@ -121,6 +121,7 @@ export UNSHARE="@UNSHARE@"
 export USERADD="@USERADD@"
 export USERDEL="@USERDEL@"
 export USERMOD="@USERMOD@"
+export UUIDGEN="@UUIDGEN@"
 export VMSTAT="@VMSTAT@"
 export WAIT="@WAIT@"
 export WC="@WC@"