Increasing ufs.dirhash_maxmem by default

[Ivan Voras]

hi,

Several people have worked on dirhash in the past so I'm posting here 
instead of individually pinging them.

The default dirhash_maxmem is currently set as 2 MB, which while may be 
sufficient some time ago it certainly isn't now. I've had to increase it 
on practically all non-trivial servers and even high-end desktops, and 
there are occasional reports on the lists that suggest it's a fairly 
common thing.

What I'd like to do is either:

1) Simply increase the default to e.g. 32 MB (trivial change) or
2) Make it a function of hibufspace (e.g. 1/32th of it, capped at 64 MB) 
which is itself autotuned. This would happen in ufsdirhash_init().

The current incarnation of dirhash has a vm_lowmem handler so it doesn't 
look like it could starve a system if overtuned.

Ideas? Objections?