[GFS2] split gfs2_dinode into on-disk and host variants

The latter is used as part of gfs2-private part of struct inode.
It actually stores a lot of fields differently; for now the
declaration is just cloned, inode field is swtiched and changes
propagated.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>