A 4GB, 4-way Opteron would create the smallest size table (16 entries) because
pidhash_init is called before mem_init which is where x86-64 sets up max_pfn.

nr_kernel_pages is setup by paging_init, called from setup_arch, which is also
where i386 sets up max_pfn.

So export nr_kernel_pages, nr_all_pages.  Use nr_kernel_pages when sizing the
PID hash.  This fixes the problem.

This also makes the pid hash dependant on the size of ZONE_NORMAL instead of
total size of memory.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>