crypto: algif_skcipher - Do not dereference ctx without socket lock

crypto: algif_skcipher - Do not dereference ctx without socket lock

commit 6454c2b83f719057069777132b13949e4c6b6350 upstream.

Any access to non-constant bits of the private context must be
done under the socket lock, in particular, this includes ctx->req.

This patch moves such accesses under the lock, and fetches the
tfm from the parent socket which is guaranteed to be constant,
rather than from ctx->req.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[bwh: Backported to 3.2:
 - Drop changes to skcipher_recvmsg_async
 - s/skcipher/ablkcipher/ in many places
 - s/skc->skcipher/skc->base/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>