[XFS] xfs_unmountfs should return void

xfs_unmounts can't and shouldn't return errors so declare it as returning
void.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31833a

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