blk_crypto_rq_bio_prep() assumes its gfp_mask argument always includes
__GFP_DIRECT_RECLAIM, so that the mempool_alloc() will always succeed.

However, blk_crypto_rq_bio_prep() might be called with GFP_ATOMIC via
setup_clone() in drivers/md/dm-rq.c.

This case isn't currently reachable with a bio that actually has an
encryption context.  However, it's fragile to rely on this.  Just make
blk_crypto_rq_bio_prep() able to fail.
Suggested-by: NSatya Tangirala <satyat@google.com>
Signed-off-by: NEric Biggers <ebiggers@google.com>
Reviewed-by: NMike Snitzer <snitzer@redhat.com>
Reviewed-by: NSatya Tangirala <satyat@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>