proc: implement proc_single_file_operations

Currently many /proc/pid files use a crufty precursor to the current seq_file
api, and they don't have direct access to the pid_namespace or the pid of for
which they are displaying data.

So implement proc_single_file_operations to make the seq_file routines easy to
use, and to give access to the full state of the pid of we are displaying data
for.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/proc_fs.h

@@ -262,6 +262,9 @@ extern void kclist_add(struct kcore_list *, void *, size_t);
 union proc_op {
 	int (*proc_get_link)(struct inode *, struct dentry **, struct vfsmount **);
 	int (*proc_read)(struct task_struct *task, char *page);
+	int (*proc_show)(struct seq_file *m,
+		struct pid_namespace *ns, struct pid *pid,
+		struct task_struct *task);
 };
 
 struct proc_inode {