fix confusion around GFP_* flags and blk_get_request

[Christoph Hellwig <hch@xxxxxx>]

Hi all,

this series sorts out the mess around how we use gfp flags in the
block layer get_request interface.

Changes since RFC:
  - don't switch to GFP_NOIO for allocations in blk_get_request.
    blk_get_request is used by the multipath code in potentially dead lock
    prone areas, so this will need a separate audit and maybe a flag.