Mount writer count and read-only bind mounts

The largest change from last time is in patch 25/26. It has been reworked to use per-cpu lock instead of a per-node one. See the patch for more details.

I would also appreciate any close review of patch 08/27. It touches a lot of code, and I'm a bit worried that a buglet or two could have snuck in there.

Note that there are a few stragglers left using IS_RDONLY() in reiser4 and in some other low-level filesystem code. The reiser4 asserts should be just fine left the way they are, plus reiser4 needs quite a bit of work before merging anyway. The ntfs usage appears to be internal, and not related to user activity.

This patch survives running ltp as well as a home-grown set of filesystem operations here:

	http://sr71.net/~dave/linux/robind-test.sh

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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.