userns: Make seq_file's user namespace accessible

struct file already has a user namespace associated with it
in file->f_cred->user_ns, unfortunately because struct
seq_file has no struct file backpointer associated with
it, it is difficult to get at the user namespace in seq_file
context. Therefore add a helper function seq_user_ns to return
the associated user namespace and a user_ns field to struct
seq_file to be used in implementing seq_user_ns.

Cc: Al Viro <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Acked-by: David S. Miller <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

fs/seq_file.c

···
 #include <linux/export.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
+#include <linux/cred.h>
 
 #include <asm/uaccess.h>
 #include <asm/page.h>
···
 	memset(p, 0, sizeof(*p));
 	mutex_init(&p->lock);
 	p->op = op;
+#ifdef CONFIG_USER_NS
+	p->user_ns = file->f_cred->user_ns;
+#endif
 
 	/*
 	 * Wrappers around seq_open(e.g. swaps_open) need to be

include/linux/seq_file.h

···
 struct path;
 struct inode;
 struct dentry;
+struct user_namespace;
 
 struct seq_file {
 	char *buf;
···
 	struct mutex lock;
 	const struct seq_operations *op;
 	int poll_event;
+#ifdef CONFIG_USER_NS
+	struct user_namespace *user_ns;
+#endif
 	void *private;
 };
 
···
 			unsigned long long num);
 int seq_put_decimal_ll(struct seq_file *m, char delimiter,
 			long long num);
+
+static inline struct user_namespace *seq_user_ns(struct seq_file *seq)
+{
+#ifdef CONFIG_USER_NS
+	return seq->user_ns;
+#else
+	extern struct user_namespace init_user_ns;
+	return &init_user_ns;
+#endif
+}
 
 #define SEQ_START_TOKEN ((void *)1)
 /*