proc: Use d_set_d_op in the namespace file descriptor code

Without this none of the pid_dentry_operations methods will
be called and we can slowly leak namespace file descriptors.

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

fs/proc/namespaces.c

diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index be177f7..d6c078e 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -54,7 +54,7 @@
 	ei->ns_ops    = ns_ops;
 	ei->ns	      = ns;
 
-	dentry->d_op = &pid_dentry_operations;
+	d_set_d_op(dentry, &pid_dentry_operations);
 	d_add(dentry, inode);
 	/* Close the race of the process dying before we return the dentry */
 	if (pid_revalidate(dentry, NULL))