fs/proc/namespaces.c: prevent crash when ns_entries[] is empty

If CONFIG_NET_NS, CONFIG_UTS_NS and CONFIG_IPC_NS are disabled,
ns_entries[] becomes empty and things like
ns_entries[ARRAY_SIZE(ns_entries) - 1] will explode.

Reported-by: Richard Weinberger <richard@nod.at>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/namespaces.c

@@ -156,15 +156,15 @@ static struct dentry *proc_ns_dir_lookup(struct inode *dir,
 	if (!ptrace_may_access(task, PTRACE_MODE_READ))
 		goto out;
 
-	last = &ns_entries[ARRAY_SIZE(ns_entries) - 1];
-	for (entry = ns_entries; entry <= last; entry++) {
+	last = &ns_entries[ARRAY_SIZE(ns_entries)];
+	for (entry = ns_entries; entry < last; entry++) {
 		if (strlen((*entry)->name) != len)
 			continue;
 		if (!memcmp(dentry->d_name.name, (*entry)->name, len))
 			break;
 	}
 	error = ERR_PTR(-ENOENT);
-	if (entry > last)
+	if (entry == last)
 		goto out;
 
 	error = proc_ns_instantiate(dir, dentry, task, *entry);