[GFS2] Shrink gfs2_inode (2) - di_major/di_minor

This removes the device numbers from this structure by using
inode->i_rdev instead. It also cleans up the code in gfs2_mknod.
It results in shrinking the gfs2_inode by 8 bytes.

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