[XFS] clean up xfs_swapext

- stop using vnodes
- use proper multiple label goto unwinding
- give the struct file * variables saner names

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30366a

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