procfs: Use the proc generic infrastructure for proc/self.

I had visions at one point of splitting proc into two filesystems.  If
that had happened proc/self being the the part of proc that actually deals
with pids would have been a nice cleanup.  As it is proc/self requires
a lot of unnecessary infrastructure for a single file.

The only user visible change is that a mounted /proc for a pid namespace
that is dead now shows a broken proc symlink, instead of being completely
invisible.  I don't think anyone will notice or care.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/proc/root.c

@@ -169,6 +169,7 @@ void __init proc_root_init(void)
 		return;
 	}
 
+	proc_self_init();
 	proc_symlink("mounts", NULL, "self/mounts");
 
 	proc_net_init();