split and move block_dev.c

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens,

block_dev.c has been mis-placed in the fs/ directory forever, and
also contains a weird mix.  This series splits it into two files,
and moves both of them to block.  The second week of the -rc1 window
might be the best time to do something like this to avoid conflicts.