dm-crypt: allow unaligned bio_vecs for direct io

Many storage devices can handle DMA for data that is not aligned to the
logical block size. The block and filesystem layers have introduced
updates to allow that kind of memory alignment flexibility when
possible.

dm-crypt, however, currently constrains itself to aligned memory because
it sends a single scatterlist element for the in/out list to the encrypt
and decrypt algorithms. This forces applications that have unaligned
data to copy through a bounce buffer, increasing CPU and memory
utilization.

Use multiple scatterlist elements to relax the memory alignment
requirement. To keep this simple, this more flexible constraint is
enabled only for certain encryption and initialization vector types,
specifically the ones that don't have additional use for the request
scatterlist elements beyond pointing to user data.

In the unlikely case where the incoming bio uses a highly fragmented
vector, the four inline scatterlist elements may not be enough, so
allocate a temporary scatterlist when needed, falling back to a mempool
for the in and out buffers to guarantee forward progress if the initial
allocation fails.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>