[XFS] move freeing the mount structure from xfs_mount_free into the callers

In the next patch we need to look at the mount structure until just before
it's freed, so we need to be able to free it as the very last thing in
xfs_unmount.

SGI-PV: 969608
SGI-Modid: xfs-linux-melb:xfs-kern:29501a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>