xfs: clean up the error handling in xfs_swap_extents

Make sure we release resources properly if we cannot clean out the COW
extents in preparation for an extent swap.

Fixes: 96987eea

 ("xfs: cancel COW blocks before swapext")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>