Soften pruning threshold on not evictable metadata

Previous code pruned 10% of dnodes once 3/4 of metadata appeared
unevictable. On workloads with many millions of dnodes and little
other metadata it creates significant load spikes for many seconds
straight. This change instead gradually increases pruning as
unevictable metadata grow above the 3/4, which may allow it to
stabilize at some level.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16401