[XFS] don't call xfs_freesb from xfs_unmountfs

xfs_readsb is called before xfs_mount so xfs_freesb should be called after
xfs_unmountfs, too. This means it now happens after a few things during
the of xfs_unmount which all have nothing to do with the superblock.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31835a

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