Tweak rebuild in-flight hard limit

Vendor testing shows we should be able to get a little more
performance if we further relax the hard limit which we're hitting.

Authored-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15324