ocfs2: Unify the setting of extended attributes

ocfs2 can store extended attributes in many ways. They can live inside the inode's inline data area, they can be stored in a single external block, or they can be in a "bucket" hanging off of a lookup tree. There are differences in how each storage type manages the attributes, and the current ocfs2 code reflects this.

There are two entirely separate code paths for setting extended attributes. The first code path handles "block" storage - the inline inode or the external block. The second handles "bucket" storage. They do very similar things, but they go about them in very different ways. This makes reading and understanding the ocfs2 xattr code harder than it needs to be. Worse, updating the xattr code requires understanding and modifying both paths.

This patch series unifies the xattr set code such that inodes, block, and buckets make the same call. It removes most of the redundant code, like the repeated implementations of truncating externally stored values.

The core of the implementation is the ocfs2_xa_loc structure. This is an in-memory representation of an ocfs2_xattr_entry. It has operations that allow blocks and buckets to behave differently during the xattr modification process. Blocks and buckets are different enough that ocfs2_xa_loc_operations has ten different ops! But with these differences encapsulated, we can have a single code path to modify an entry.

The changes don't reduce the size of the source code perceptibly (~10 lines of actual code without comments or blanks), but to my mind they greatly improve the readability and maintainability.

The current series is based on the 'fixes' branch; I'll need to rebase it atop cachme before it can go upstream. I wanted to send it out as it stands now so that the concept can be reviewed while I work on merge-window.

Tao and Tiger, I'd really appreciate you going over the changes with a find-toothed comb. Make sure I have my ideas about block and bucket stuff correct, etc. Let me know if you think this is a stupid change, too :-)