nvme: target: fix nvme_keyring_id() references

In configurations without CONFIG_NVME_TARGET_TCP_TLS, the keyring
code might not be available, or using it will result in a runtime
failure:

x86_64-linux-ld: vmlinux.o: in function `nvmet_ports_make':
configfs.c:(.text+0x100a2110): undefined reference to `nvme_keyring_id'

Add a check to ensure we only check the keyring if there is a chance
of it being used, which avoids both the runtime and link-time
problems.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20231122224719.4042108-2-arnd@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/target/configfs.c

@@ -1893,7 +1893,7 @@ static struct config_group *nvmet_ports_make(struct config_group *group,
 		return ERR_PTR(-ENOMEM);
 	}
 
-	if (nvme_keyring_id()) {
+	if (IS_ENABLED(CONFIG_NVME_TARGET_TCP_TLS) && nvme_keyring_id()) {
 		port->keyring = key_lookup(nvme_keyring_id());
 		if (IS_ERR(port->keyring)) {
 			pr_warn("NVMe keyring not available, disabling TLS\n");