MFC of 253973:

To better understand performance problems with journalled soft updates,
we need to collect the highest level of allocation for each of the
different soft update dependency structures. This change collects these
statistics and makes them available using `sysctl debug.softdep.highuse'.

Reviewed by: kib
Tested by: Peter Holm

MFC of 253974:

With the addition of journalled soft updates, the "newblk" structures
persist much longer than previously. Historically we had at most 100
entries; now the count may reach a million. With the increased count
we spent far too much time looking them up in the grossly undersized
newblk hash table. Configure the newblk hash table to accurately reflect
the number of entries that it must index.

Reviewed by: kib
Tested by: Peter Holm
MFC after: 2 weeks