xfs: fix the check for COW extents in xfs_swap_extents

i_cnextents does not include delayed allocated extents, so switch
to the inode fork size check that we already use in other places
instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>