ZIL: Force writing of open LWB on suspend

Under parallel workloads ZIL may delay writes of open LWBs that
are not full enough. On suspend we do not expect anything new to
appear since zil_get_commit_list() will not let it pass, only
returning TXG number to wait for. But I suspect that waiting for
the TXG commit without having the last LWB issued may not wait for
its completion, resulting in panic described in #17509.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #17521