nvme-fabrics: replace deprecated strncpy with strscpy

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We expect both data->subsysnqn and data->hostnqn to be NUL-terminated
based on their usage with format specifier ("%s"):
fabrics.c:
322: dev_err(ctrl->device,
323:   "%s, subsysnqn \"%s\"\n",
324:   inv_data, data->subsysnqn);
...
349: dev_err(ctrl->device,
350: 	 "Connect for subsystem %s is not allowed, hostnqn: %s\n",
351: 	 data->subsysnqn, data->hostnqn);

Moreover, there's no need to NUL-pad since `data` is zero-allocated
already in fabrics.c:
383: data = kzalloc(sizeof(*data), GFP_KERNEL);
... therefore any further NUL-padding is rendered useless.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

I opted not to switch NVMF_NQN_SIZE to sizeof(data->xyz) because the
size is defined as:
|       /* NQN names in commands fields specified one size */
|       #define NVMF_NQN_FIELD_LEN	256

... while NVMF_NQN_SIZE is defined as:
|       /* However the max length of a qualified name is another size */
|       #define NVMF_NQN_SIZE		223

Since 223 seems pretty magic, I'm not going to touch it.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: KSPP#90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231018-strncpy-drivers-nvme-host-fabrics-c-v1-1-b6677df40a35@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: JustinStitt

drivers/nvme/host/fabrics.c

@@ -387,8 +387,8 @@ static struct nvmf_connect_data *nvmf_connect_data_prep(struct nvme_ctrl *ctrl,
 
 	uuid_copy(&data->hostid, &ctrl->opts->host->id);
 	data->cntlid = cpu_to_le16(cntlid);
-	strncpy(data->subsysnqn, ctrl->opts->subsysnqn, NVMF_NQN_SIZE);
-	strncpy(data->hostnqn, ctrl->opts->host->nqn, NVMF_NQN_SIZE);
+	strscpy(data->subsysnqn, ctrl->opts->subsysnqn, NVMF_NQN_SIZE);
+	strscpy(data->hostnqn, ctrl->opts->host->nqn, NVMF_NQN_SIZE);
 
 	return data;
 }