[PATCH] fix handling of st_nlink on procfs root

1) it should use nr_processes(), not nr_threads; otherwise we are getting
very confused find(1) and friends, among other things.
2) better do that at stat() time than at every damn lookup in procfs root.

Patch had been sitting in FC4 kernels for many months now...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>