blk-crypto: use mempool_alloc_bulk for encrypted bio page allocation

blk-crypto: use mempool_alloc_bulk for encrypted bio page allocation

Calling mempool_alloc in a loop is not safe unless the maximum allocation
size times the maximum number of threads using it is less than the
minimum pool size.  Use the new mempool_alloc_bulk helper to allocate
all missing elements in one pass to remove this deadlock risk.  This
also means that non-pool allocations now use alloc_pages_bulk which can
be significantly faster than a loop over individual page allocations.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>