[GFS2] Shrink gfs2_inode (1) - di_header/di_num

The metadata header doesn't need to be stored in the incore
struct gfs2_inode since its constant, and this patch removes it.
Also, there is already a field for the inode's number in the
struct gfs2_inode, so we don't need one in struct gfs2_dinode_host
as well.

This saves 28 bytes of space in the struct gfs2_inode.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>