mempool_alloc_bulk and various mempool improvements

[Christoph Hellwig]

Hi all,

this series adds a bulk version of mempool_alloc that makes allocating
multiple objects deadlock safe.

The initial users is the blk-crypto-fallback code:

  https://lore.kernel.org/linux-block/20251031093517.1603379-1-hch@xxxxxx/

with which v1 was posted, but I also have a few other users in mind.

Changes since v1:
 - fix build for !CONFIG_FAULT_INJECTION
 - improve the kerneldoc comments further
 - refactor the code so that the mempool_alloc fastpath does not
   have to deal with arrays
 - don't support !__GFP_DIRECT_RECLAIM for bulk allocations
 - do poll allocations even if not all elements are available
 - s/elem/elems/
 - use a separate failure injection know for the bulk allocator

diffstat:
 include/linux/fault-inject.h |    8 -
 include/linux/mempool.h      |    7 
 mm/mempool.c                 |  341 ++++++++++++++++++++++++++++---------------
 3 files changed, 240 insertions(+), 116 deletions(-)