Replay logs before starting ztest workers

Replay logs before starting ztest workers

This patch ensures that logs are replayed on all datasets prior
to starting ztest workers. This ensures that the call to
vdev_offline() a log device in ztest_fault_inject() will not fail
due to the log device being required for replay.

This patch also fixes a small issue found during testing where
spa_keystore_load_wkey() does not check that the dataset specified
is an encryption root. This check was present in libzfs, however.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8084