lv-to-bcache: grow the lv if the data can't shrink

[g2p]

No description provided.

[marcan]

Particularly if the superblock isn't supported. I was hoping to convert some LVs to bcache, but they're VM disk images, not raw filesystems, so blocks doesn't know what to do with them. I was expecting it to grow and rotate the LV and not have to care about the underlying data.