xfs: don't allow realtime swap files

xfs: don't allow realtime swap files

Linux swapfiles use bmap which has no ability to tell the swap code
that the blocks its reporting aren't actually on the data device.
Therefore, swapon on a realtime file could corrupt random blocks on
the data device, so we must ensure that swapon cannot happen.

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