remove block layer bounce buffering for MMC

[Christoph Hellwig]

Hi everyone,

this series converts the remaining MMC host drivers to properly kmap the
scatterlist entries it does PIO operations on, and then goes on to
remove the usage of block layer bounce buffering (which I plan to remove
eventually) from the MMC layer.

As a bonus I've converted various drivers to the proper scatterlist
helpers so that at least in theory they are ready for chained
scatterlists.

All the changes are compile tested only as I don't have any of the
hardware, so a careful review would be appreciated.