Increase meta-dnode redundancy in "some" mode

Loss of one indirect block of the meta dnode likely means loss of
the whole dataset. It is worse than one file that the man page
promises, and in my opinion is not much better than "none" mode.

This change restores redundancy of the meta-dnode indirect blocks,
while same time still corrects expectations in the man page.

Reviewed-by: Akash B <akash-b@hpe.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #17339