Linux: Make zfs_prune() fair on NUMA systems

Previous code evicted nr_to_scan items from each NUMA node. This
not only multiplied the eviction by the number of nodes, but could
exhaust the smaller ones, evicting inodes used by acive workload
and requiring their immediate recreation. This patch spreads the
requested eviction between all NUMA nodes proportionally to their
evictable counts, which should be closer to expected LRU logic.
See kernel's super_cache_scan() as a similar logic example.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16397