If UFS or LFS dirhash is enabled in the kernel, set the dirhash cache
size dependant on memory size.  If less than 128MB of memory, default
to no cache.  With 128MB of memory or more, use a maximum cache size of
1/64th of memory; cap maximum default cache size to 32MB (for systems
with 2GB of memory or more).

The dirhash cache sizes are still explicityly setable by sysctl(8) or
by adding relevant entry(s) to sysctl.conf(5).

(simonb)