[XFS] implement generic xfs_btree_updkey

From: Dave Chinner <dgc@sgi.com>

Note that there are many > 80 char lines introduced due to the
xfs_btree_key casts. But the places where this happens is throw-away code
once the whole btree code gets merged into a common implementation.

The same is true for the temporary xfs_alloc_log_keys define to the new
name. All old users will be gone after a few patches.

[hch: split out from bigger patch and minor adaptions]

SGI-PV: 985583

SGI-Modid: xfs-linux-melb:xfs-kern:32193a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Bill O'Donnell <billodo@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>