[XFS] kill xfs_dinode_core_t

Now that we have a separate xfs_icdinode_t for the in-core inode which
gets logged there is no need anymore for the xfs_dinode vs xfs_dinode_core
split - the fact that part of the structure gets logged through the inode
log item and a small part not can better be described in a comment.

All sizeof operations on the dinode_core either really wanted the
icdinode and are switched to that one, or had already added the size
of the agi unlinked list pointer.  Later both will be replaced with
helpers once we get the larger CRC-enabled dinode.

Removing the data and attribute fork unions also has the advantage that
xfs_dinode.h doesn't need to pull in every header under the sun.

While we're at it also add some more comments describing the dinode
structure.

(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>