[XFS] kill xfs_ialloc_log_di

xfs_ialloc_log_di is only used to log the full inode core + di_next_unlinked.
That means all the offset magic is not nessecary and we can simply use
xfs_trans_log_buf directly.  Also add a comment describing what we should do
here instead.

(First sent on October 7th)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>