crypto: krb5 - filter out async aead implementations at alloc

crypto: krb5 - filter out async aead implementations at alloc

krb5_aead_encrypt(), krb5_aead_decrypt() in rfc3961_simplified.c and
rfc8009_encrypt(), rfc8009_decrypt() in rfc8009_aes2.c set a NULL
completion callback and treat any negative return from
crypto_aead_{encrypt,decrypt}() as terminal, falling through to
kfree_sensitive(buffer).  When the encrypt_name resolves to an
async AEAD instance the request returns -EINPROGRESS, the buffer
is freed while the backend's worker still holds a pointer, and the
worker dereferences the freed slab on completion.

KASAN report under UML+SLUB with a synthetic async aead backend
bound to krb5->encrypt_name:

  BUG: KASAN: slab-use-after-free in t5_stub_complete+0x7d/0xc7

The helpers were written synchronously, so filter the async
instances out at allocation time instead of plumbing
crypto_wait_req() through every call site.

Reachable via net/rxrpc/rxgk.c, fs/afs/cm_security.c and
net/ceph/crypto.c on systems with an async AEAD provider bound to
the krb5 enctype name.

Fixes: 00244da40f78 ("crypto/krb5: Implement the Kerberos5 rfc3961 encrypt and decrypt functions")
Fixes: 6c3c0e86c2ac ("crypto/krb5: Implement the AES enctypes from rfc8009")
Cc: stable@vger.kernel.org
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>