xfs: refactor swapext code

Refactor the swapext function to pull out the fork swapping piece
into a separate function.  In the next patch we'll add in the bit
we need to make it work with rmap filesystems.

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