Fix zpool create -t ENOENT bug.

In userland we need to switch over to the temporary name once the
pool has been created, otherwise the root dataset won't mount
and the error "cannot open 'the_real_name': dataset does not exist"
is printed.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2760