make proc_ns_operations work with struct ns_common * instead of void *

We can do that now.  And kill ->inum(), while we are at it - all instances
are identical.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/inode.c

@@ -33,7 +33,7 @@ static void proc_evict_inode(struct inode *inode)
 	struct proc_dir_entry *de;
 	struct ctl_table_header *head;
 	const struct proc_ns_operations *ns_ops;
-	void *ns;
+	struct ns_common *ns;
 
 	truncate_inode_pages_final(&inode->i_data);
 	clear_inode(inode);