xfs: introduce inode log format object

We currently carry around and log an entire inode core in the
struct xfs_inode. A lot of the information in the inode core is
duplicated in the VFS inode, but we cannot remove this duplication
of infomration because the inode core is logged directly in
xfs_inode_item_format().

Add a new function xfs_inode_item_format_core() that copies the
inode core data into a struct xfs_icdinode that is pulled directly
from the log vector buffer. This means we no longer directly
copy the inode core, but copy the structures one member at a time.
This will be slightly less efficient than copying, but will allow us
to remove duplicate and unnecessary items from the struct xfs_inode.

To enable us to do this, call the new structure a xfs_log_dinode,
so that we know it's different to the physical xfs_dinode and the
in-core xfs_icdinode.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>