swiotlb cleanups v3

[Christoph Hellwig]

Hi Konrad,

this series contains a bunch of swiotlb cleanups, mostly to reduce the
amount of internals exposed to code outside of swiotlb.c, which should
helper to prepare for supporting multiple different bounce buffer pools.

Changes since v2:
 - fix a bisetion hazard that did not allocate the alloc_size array
 - dropped all patches already merged

Changes since v1:
 - rebased to v5.12-rc1
 - a few more cleanups
 - merge and forward port the patch from Claire to move all the global
   variables into a struct to prepare for multiple instances