xfs: remove XFS_IO_INVALID

The invalid state isn't any different from a hole, so merge the two
states.  Use the more descriptive hole name, but keep it as the first
value of the enum to catch uninitialized fields.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>