Fix vdev_initialize_restart / removal race

Resolve a vdev_initialize crash uncovered by ztest. Similar
to when starting a new initialization verify that a removal
is not in progress. Additionally, do not restart when the
thread already exists. This check is now congruent with the
POOL_INITIALIZE_DO handling in spa_vdev_initialize_impl().

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8477