xfs: fix direction in XFS_IOC_EXCHANGE_RANGE

The kernel reads userspace's buffer but does not write it back.
Therefore this is really an _IOW ioctl.  Change this before 6.10 final
releases.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>