fs/proc: calculate /proc/* and /proc/*/task/* nlink at init time

Runtime nlink calculation works but meh.  I don't know how to do it at
compile time, but I know how to do it at init time.

Shift "2+" part into init time as a bonus.

Link: http://lkml.kernel.org/r/20161122195549.GB29812@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 8d3e484..4bd0373 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -122,6 +122,7 @@ void __init proc_root_init(void)
 	int err;
 
 	proc_init_inodecache();
+	set_proc_pid_nlink();
 	err = register_filesystem(&proc_fs_type);
 	if (err)
 		return;