[XFS] remove the mount inode list

Now we've removed all users of the mount inode list, we can kill it. This
reduces the size of the xfs_inode by 2 pointers.

SGI-PV: 988139

SGI-Modid: xfs-linux-melb:xfs-kern:32293a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>