ztest: creates partially initialized root dataset

Since d8fdfc2 was integrated dsl_pool_create() does not call
dmu_objset_create_impl() for the root dataset when running in
userland (ztest): this creates a pool with a partially initialized
root dataset. Trying to import and use this pool results in both
zpool and zfs executables dumping core.

Fix this by adopting an alternative change suggested in OpenZFS 8607
code review.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Tom Caputi <tcaputi@datto.com>
Original-patch-by: Robert Mustacchi <rm@joyent.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8277