crypto: api - Fix use-after-free and race in crypto_spawn_alg

There are two problems in crypto_spawn_alg.  First of all it may
return spawn->alg even if spawn->dead is set.  This results in a
double-free as detected by syzbot.

Secondly the setting of the DYING flag is racy because we hold
the read-lock instead of the write-lock.  We should instead call
crypto_shoot_alg in a safe manner by gaining a refcount, dropping
the lock, and then releasing the refcount.

This patch fixes both problems.

Reported-by:  <syzbot+fc0674cde00b66844470@syzkaller.appspotmail.com>
Fixes: 4f87ee11 ("crypto: api - Do not zap spawn->alg")
Fixes: 73669cc5

 ("crypto: api - Fix race condition in...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>