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