nvme: keyring: fix conditional compilation

The keyring and auth functions can be called from both the host and
the target side and are controlled by Kconfig options for each of the
combinations, but the declarations are controlled by #ifdef checks
on the shared Kconfig symbols.

This leads to link failures in combinations where one of the frontends
is built-in and the other one is a module, and the keyring code
ends up in a module that is not reachable from the builtin code:

ld: drivers/nvme/host/core.o: in function `nvme_core_exit':
core.c:(.exit.text+0x4): undefined reference to `nvme_keyring_exit'
ld: drivers/nvme/host/core.o: in function `nvme_core_init':
core.c:(.init.text+0x94): undefined reference to `nvme_keyring_init

ld: drivers/nvme/host/tcp.o: in function `nvme_tcp_setup_ctrl':
tcp.c:(.text+0x4c18): undefined reference to `nvme_tls_psk_default'

Address this by moving nvme_keyring_init()/nvme_keyring_exit() into
module init/exit functions for the keyring module.

Fixes: be8e82caa6859 ("nvme-tcp: enable TLS handshake upcall")
Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>

nvme: common: make keyring and auth separate modules

When only the keyring module is included but auth is not, modpost
complains about the lack of a module license tag:

ERROR: modpost: missing MODULE_LICENSE() in drivers/nvme/common/nvme-common.o

Address this by making both modules buildable standalone,
removing the now unnecessary CONFIG_NVME_COMMON symbol
in the process.

Also, now that NVME_KEYRING config symbol can be either a module or
built-in, the stubs need to check for '#if IS_ENABLED' rather than a
simple '#ifdef'.

Fixes: 9d77eb5277849 ("nvme-keyring: register '.nvme' keyring")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>

nvme-keyring: implement nvme_tls_psk_default()

Implement a function to select the preferred PSK for TLS.
A 'retained' PSK should be preferred over a 'generated' PSK,
and SHA-384 should be preferred to SHA-256.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>

nvme-keyring: register '.nvme' keyring

Register a '.nvme' keyring to hold keys for TLS and DH-HMAC-CHAP and
add a new config option NVME_KEYRING.
We need a separate keyring for NVMe as the configuration is done
via individual commands (eg for configfs), and the usual per-session
or per-process keyrings can't be used.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>