procfs: switch /proc/self away from proc_dir_entry

Just have it pinned in dcache all along and let procfs ->kill_sb()
drop it before kill_anon_super().

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

include/linux/pid_namespace.h

@@ -28,6 +28,7 @@ struct pid_namespace {
 	struct pid_namespace *parent;
 #ifdef CONFIG_PROC_FS
 	struct vfsmount *proc_mnt;
+	struct dentry *proc_self;
 #endif
 #ifdef CONFIG_BSD_PROCESS_ACCT
 	struct bsd_acct_struct *bacct;