xfs: move swap extent code to xfs_extent_ops

xfs: move swap extent code to xfs_extent_ops

Swapping extents is clearly an extent operaiton, and it is not
shared with userspace. Move the code to xfs_extent_ops.[ch], and
the userspace ioctl structure definition to xfs_fs.h where most of
the other ioctl structure definitions are. The means xfs_dfrag.h is
no longer needed in userspace.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>