Patch from Bill Irwin.   Prodding from me.

The hashtables in kernel/pid.c are 128 kbytes, which is far too large for
very small machines.

So we dynamically size them and allocate them from bootmem.  From 16 buckets
on the very smallest machine up to 4096 buckets (effectively half the current
size) with one gigabyte of memory or more.

The patch also switches the hashing from a custom hash over to the more
powerful hash_long().