generic btree implementation, version 3

Firstly all but one of Dave's review comments are addressed. There is also a new first patch to clean up the defintion of struct xfs_btree_block which I would consider an ASAP merge candidate.

But there are also some bigger changes. For reimplemented the newroot and killroot methods. In both cases one method was used for block rooted and inode rooted btree but doing something quite different. This patchset gets rid of these two method completely and implements both cases in xfs_btree.c. For new_root that was as easy as just calling xfs_btree_new_root directly for the !XFS_BTREE_ROOT_IN_INODE and moving xfs_bmbt_newroot into the common code and calling it for the other case. The XFS_BTREE_ROOT_IN_INODE case for kill_root was similarly easily solved by just moving the code around and making it block/rec/key/ptr size agnostic, but the !XFS_BTREE_ROOT_IN_INODE needed a little care.

The alloc and inobt trees were in fact using the almost exact sequence built from ->set_root and ->free_block but the odd way to pass the block to be freed made this non-obvious. By changing the calling convention to the normal one this one could be unified. I've left in some nasty asserts to ensure the assumptions about these blocks wasn't wrong.

The other big news is the last patch in the series, which adds rec_len and key_len members to the btree_ops structure and uses this to make the ptr_addr, key_addr, rec_addr, set_key, set_rec, move_keys, move_recs, copy_keys, copy_recsm log_keys and log_recs methods generic. This patch by itselfs saves a few hundred lines of code and more than two kilobytes in the binary image. With this eventual worse generated code that needs to multiply by variables instead of constants should easily be offset by the reduced instruction cache footprint. The patch is so far not really comments and more a proof of concept - if everyone agrees with this approach the changes will be merged into the earlier patches.

Now even with this move the set_*, move_* and copy_* interfaces are left as-is. All the old discussion still applies except that things might be a little more clear now that there's just one implementation each and everything is contained in one file. I think keeping the copy_* interfaces is a good idea, they are now basically just typed memcpy wrappers - although we should switch the order of the src and dst arguments to be the same as memcpy. The set_* interfaces can probably go away - over copy_ they just add the index based addressing which most callers don't use anyway. I'm not sure what to do with move_* - these are the most ugly helpers, so maybe we should just make them memmove wrappers in the style of copy_ and leave all addressing to the callers.

With all these changes the stats for this series are now:

 37 files changed, 6309 insertions(+), 7873 deletions(-)

   text	   data	    bss	    dec	    hex	filename
 631577	   4227	   3092	 638896	  9bfb0	fs/xfs/xfs.ko.base
 614298	   4435	   3124	 621857	  97d21	fs/xfs/xfs.ko.btree