nvmet-tcp: Ensure old keys are freed before replacing new ones

Previously after the host sends a REPLACETLSPSK we freed the TLS keys as
part of calling nvmet_auth_sq_free() on success. A recent change ensured
we don't free the keys, allowing REPLACETLSPSK to work.

But that fix results in a kernel memory leak when running

```
nvme_trtype=loop ./check nvme/041 nvme/042 nvme/043 nvme/044 nvme/045 nvme/051 nvme/052
echo scan > /sys/kernel/debug/kmemleak
cat /sys/kernel/debug/kmemleak
```

We can't free the keys on a successful DHCHAP operation, otherwise the
next REPLACETLSPSK will fail, so instead let's free them before we
replace them as part of nvmet_auth_challenge().

This ensures that REPLACETLSPSK works, while also avoiding any memory
leaks.

Fixes: 2e6eb6b ("nvmet-tcp: Don't free SQ on authentication success")
Signed-off-by: Alistair Francis <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>

Author: alistair23

drivers/nvme/target/fabrics-cmd-auth.c

@@ -412,6 +412,13 @@ static int nvmet_auth_challenge(struct nvmet_req *req, void *d, int al)
 	int hash_len = nvme_auth_hmac_hash_len(ctrl->shash_id);
 	int data_size = sizeof(*d) + hash_len;
 
+	/*
+	 * If replacing the keys then we have previous successful keys
+	 * that might be leaked, so we need to free them here.
+	 */
+	if (req->sq->dhchap_c1)
+		nvmet_auth_sq_free(req->sq);
+
 	if (ctrl->dh_tfm)
 		data_size += ctrl->dh_keysize;
 	if (al < data_size) {