Add vm_lowmem event handler for dirhash. This will cause dirhashes to be

Add vm_lowmem event handler for dirhash. This will cause dirhashes to be
deleted when the system is low on memory. This ought to allow an increase to
vfs.ufs.dirhash_maxmem on machines that have lots of memory, without
degrading performance by having too much memory reserved for dirhash when
other things need it. The default value for dirhash_maxmem is being kept at
2MB for now, though.

This work was mostly done during the 2008 Google Summer of Code.

Approved by:	dwmalone (mentor), re
MFC after:	3 months