ZIL: Tune some assertions.

In zil_free_lwb() we should first assert lwb_state or the rest of
assertions can be misleading if it is false.

Add lwb_state assertions in zil_lwb_add_block() to make sure we are
not trying to add elements to lwb_vdev_tree after it was processed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15227