xfs: cleanup error handling in xfs_swap_extents

Use multiple lables for proper error unwinding and get rid of some now
superflous variables.

Signed-off-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>