Improve L2 caching control for prefetched indirects

dbuf_prefetch_impl() should look on level of current indirect, not
the target prefetch level. dbuf_prefetch_indirect_done() should
call dnode_level_is_l2cacheable() if we have dpa_dnode to pass it.
It should fix some both false positive and negative L2ARC caching.

While there, fix redacted feature activation assertions. One was
always true, while another could give false positive if dpa_dnode
is NULL.

George Amanakis <gamanakis@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #17204