crypto: algif_skcipher - force synchronous processing on trees without ctx->state

In the Linux kernel, the following vulnerability has been resolved:

crypto: algif_skcipher - force synchronous processing on trees without ctx->state

The AIO/async path in skcipher_recvmsg() passes the socket-wide ctx->iv
directly into the skcipher request. After io_submit() the socket lock is
dropped and the request is processed asynchronously, so a concurrent
sendmsg(ALG_SET_IV) can overwrite ctx->iv and make the in-flight request
run under an attacker-controlled IV. For CTR/stream modes this is
IV/keystream reuse and lets an unprivileged user recover the plaintext of
a concurrent operation.

Snapshotting ctx->iv into per-request storage for the async path is not
sufficient. For ciphers with statesize == 0 - which includes cbc and ctr -
the MSG_MORE inter-chunk IV chaining is carried solely by the in-place
req->iv writeback, which a snapshot redirects into per-request memory that
af_alg_free_resources() releases on completion, silently producing wrong
output. Writing the IV back from the completion callback instead is not
possible either: that would require lock_sock() there, but the callback can
run in softirq/atomic context, so it must not sleep.

Make the operation synchronous instead, which removes both the IV race and
any writeback race. This is equivalent to the upstream resolution, commit
fcc77d33a34c ("net: Remove support for AIO on sockets"), which removed the
AIO socket path across net/ entirely and so produces the same end state for
this file. This patch deviates from that commit deliberately: rather than
removing AIO socket support tree-wide, which would be far too invasive for
stable, it removes only the AIO branch in crypto/algif_skcipher.c.
io_submit() now completes synchronously; AF_ALG async is rarely used in
practice.

The -EIOCBQUEUED check in skcipher_recvmsg() is now dead but harmless,
and is left alone to keep the fix minimal.

Tested on 6.6.y: attacker IV injection dropped from 2296/200000 to 0/200000
after the change; MSG_MORE chunked CTR output bit-identical to single-shot.