This patch removes the loop feature wherein we remap BIOs for block-backed
loop.  So file-backed and block-backed loop are handled identically.

It cleans up the code a lot and fixes the low-on-memory lockups which
block-backed loop currently suffers.

What we lose is the journalling ordering guarantees which
exts-on-loop-on-blockdev had.  But dm-crypt provides that.