blk-crypto: use on-stack skcipher requests for fallback en/decryption

blk-crypto: use on-stack skcipher requests for fallback en/decryption

Allocating a skcipher request dynamically can deadlock or cause
unexpected I/O failures when called from writeback context.  Avoid the
allocation entirely by using on-stack skciphers, similar to what the
non-blk-crypto fscrypt path already does.

This drops the incomplete support for asynchronous algorithms, which
previously could be used, but only synchronously.

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