crypto: af_alg/hash: Fix recvmsg() after sendmsg(MSG_MORE)

If an AF_ALG socket bound to a hashing algorithm is sent a zero-length
message with MSG_MORE set and then recvmsg() is called without first
sending another message without MSG_MORE set to end the operation, an oops
will occur because the crypto context and result doesn't now get set up in
advance because hash_sendmsg() now defers that as long as possible in the
hope that it can use crypto_ahash_digest() - and then because the message
is zero-length, it the data wrangling loop is skipped.

Fix this by handling zero-length sends at the top of the hash_sendmsg()
function.  If we're not continuing the previous sendmsg(), then just ignore
the send (hash_recvmsg() will invent something when called); if we are
continuing, then we finalise the request at this point if MSG_MORE is not
set to get any error here, otherwise the send is of no effect and can be
ignored.

Whilst we're at it, remove the code to create a kvmalloc'd scatterlist if
we get more than ALG_MAX_PAGES - this shouldn't happen.

Fixes: c662b043 ("crypto: af_alg/hash: Support MSG_SPLICE_PAGES")
Reported-by:  <syzbot+13a08c0bf4d212766c3c@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/000000000000b928f705fdeb873a@google.com/


Reported-by:  <syzbot+14234ccf6d0ef629ec1a@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/000000000000c047db05fdeb8790@google.com/


Reported-by:  <syzbot+4e2e47f32607d0f72d43@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/000000000000bcca3205fdeb87fb@google.com/


Reported-by:  <syzbot+472626bb5e7c59fb768f@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/000000000000b55d8805fdeb8385@google.com/


Signed-off-by: David Howells <dhowells@redhat.com>
Reported-and-tested-by:  <syzbot+6efc50cc1f8d718d6cb7@syzkaller.appspotmail.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lore.kernel.org/r/427646.1686913832@warthog.procyon.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>