anon_inodes: mark the anon inode private

Inotify was switched to use anon_inode instead of its own private filesystem
which only had one inode in commit c44dcc56d2b5c7 "switch inotify_user to
anon_inode"

The problem with this is that now the inotify inode is not a distinct inode
which can be managed by LSMs.  userspace tools which use inotify were allowed
to use the inotify inode but may not have had permission to do read/write type
operations on the anon_inode.  After looking at the anon_inode and its users
it looks like the best solution is to just mark the anon_inode as S_PRIVATE
so the security system will ignore it.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/anon_inodes.c

@@ -209,6 +209,7 @@ static struct inode *anon_inode_mkinode(void)
 	inode->i_mode = S_IRUSR | S_IWUSR;
 	inode->i_uid = current_fsuid();
 	inode->i_gid = current_fsgid();
+	inode->i_flags |= S_PRIVATE;
 	inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
 	return inode;
 }