blk-crypto: handle the fallback above the block layer

blk-crypto: handle the fallback above the block layer

Add a blk_crypto_submit_bio helper that either submits the bio when
it is not encrypted or inline encryption is provided, but otherwise
handles the encryption before going down into the low-level driver.
This reduces the risk from bio reordering and keeps memory allocation
as high up in the stack as possible.

Note that if the submitter knows that inline enctryption is known to
be supported by the underyling driver, it can still use plain
submit_bio.

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