cleanup and refactor BLOCK_PC mapping helpers V2

[Christoph Hellwig]

This series is based on the first two patches from the reent
series that Dongsu Park recently sent, and refactors and cleanups
up various aspect of our BLOCK_PC mapping helpers.

Changes since V1:
 - keep a separate blk_rq_map_user for now, it contains magic
   for the sg and tape drivers that's still missing from
   blk_rq_map_user_iov

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