sysctl: add support for poll()

Adding support for poll() in sysctl fs allows userspace to receive
notifications of changes in sysctl entries.  This adds a infrastructure to
allow files in sysctl fs to be pollable and implements it for hostname and
domainname.

[akpm@linux-foundation.org: s/declare/define/ for definitions]
Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
Cc: Greg KH <gregkh@suse.de>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
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/utsname.h

@@ -37,6 +37,14 @@ struct new_utsname {
 #include <linux/nsproxy.h>
 #include <linux/err.h>
 
+enum uts_proc {
+	UTS_PROC_OSTYPE,
+	UTS_PROC_OSRELEASE,
+	UTS_PROC_VERSION,
+	UTS_PROC_HOSTNAME,
+	UTS_PROC_DOMAINNAME,
+};
+
 struct user_namespace;
 extern struct user_namespace init_user_ns;
 
@@ -80,6 +88,14 @@ static inline struct uts_namespace *copy_utsname(unsigned long flags,
 }
 #endif
 
+#ifdef CONFIG_PROC_SYSCTL
+extern void uts_proc_notify(enum uts_proc proc);
+#else
+static inline void uts_proc_notify(enum uts_proc proc)
+{
+}
+#endif
+
 static inline struct new_utsname *utsname(void)
 {
 	return &current->nsproxy->uts_ns->name;