Read-only bind mounts

This is against 2.6.23-rc6. Compared to 2.6.23-rc6-mm1, there are a few (trivial) merge conflicts with the audit git tree, and one slightly more complicated one with ext2-reservations.patch.

Changes from last post:
- added several kerneldoc comments for exported functions
- broke out the 'mnt' variable patch
- removed WARN_ON() in ioctl patch

The first four patches here are very simple cleanups, and can go in ahead of the rest of the patches.

If you are reviewing this, the things that need the most review are the last two patches that actually add the logic for tracking mount write counts.

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Why do we need r/o bind mounts?

This feature allows a read-only view into a read-write filesystem. In the process of doing that, it also provides infrastructure for keeping track of the number of writers to any given mount.

This has a number of uses. It allows chroots to have parts of filesystems writable. It will be useful for containers in the future because users may have root inside a container, but should not be allowed to write to somefilesystems. This also replaces patches that vserver has had out of the tree for several years.

It allows security enhancement by making sure that parts of your filesystem read-only (such as when you don't trust your FTP server), when you don't want to have entire new filesystems mounted, or when you want atime selectively updated. I've been using the following script to test that the feature is working as desired. It takes a directory and makes a regular bind and a r/o bind mount of it. It then performs some normal filesystem operations on the three directories, including ones that are expected to fail, like creating a file on the r/o mount.