cleanup and refactor BLOCK_PC mapping helpers V2

From: Christoph Hellwig
Date: Sun Jan 18 2015 - 10:17:42 EST


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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