proc: Implement /proc/thread-self to point at the directory of the current thread

/proc/thread-self is derived from /proc/self.  /proc/thread-self
points to the directory in proc containing information about the
current thread.

This funtionality has been missing for a long time, and is tricky to
implement in userspace as gettid() is not exported by glibc.  More
importantly this allows fixing defects in /proc/mounts and /proc/net
where in a threaded application today they wind up being empty files
when only the initial pthread has exited, causing problems for other
threads.

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

include/linux/pid_namespace.h

@@ -33,6 +33,7 @@ struct pid_namespace {
 #ifdef CONFIG_PROC_FS
 	struct vfsmount *proc_mnt;
 	struct dentry *proc_self;
+	struct dentry *proc_thread_self;
 #endif
 #ifdef CONFIG_BSD_PROCESS_ACCT
 	struct bsd_acct_struct *bacct;