oh the joy of swap files

[Christoph Hellwig <hch@xxxxxx>]

Quirk out of ->bmap for reflink inodes because the swap code still
believes it's a good idea to blindly bypass the fs if it exists.

This is a bit of a crude hack, but without redesigning how swapfiles
work we can't really do very much better.  Except for a real error
code from ->bmap, but that's a bit like putting lipstick on a pig..