Initialize the taskq entry embedded within struct vdev

As part of the stack reduction effort in
50b25b2187134ac7b19cf93bd35a420223f1d343, a zio_t containing a taskq_ent
was added to struct vdev_queue which itself is part of struct vdev.
The taskq entry should be initialized as is currently done in zio_create()
for newly-created bare zio_t object. The rationale is the same as is
described in f467b05a265abcfb8e5a3269f79d08f36a58646a.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3709